diff --git a/app_test.go b/app_test.go
index d566601..0781425 100644
--- a/app_test.go
+++ b/app_test.go
@@ -98,8 +98,8 @@ func TestParseCSVString(t *testing.T) {
 func TestGetCommands(t *testing.T) {
 	reg := NewCommandRegistry()
 	cmds := reg.GetCommands()
-	if len(cmds) != 17 {
-		t.Errorf("expected 12 commands, got %d", len(cmds))
+	if len(cmds) != 18 {
+		t.Errorf("expected 18 commands, got %d", len(cmds))
 	}
 	// Check that all have IDs
 	for _, cmd := range cmds {
diff --git a/commands.go b/commands.go
index 806a458..d79dbf2 100644
--- a/commands.go
+++ b/commands.go
@@ -34,6 +34,7 @@ func (c *CommandRegistry) GetCommands() []Command {
 		{ID: "sort-desc", Name: "Sort Z-A", Shortcut: ""},
 		{ID: "sort-advanced", Name: "Sort Advanced", Shortcut: ""},
 		{ID: "match-cell", Name: "Match Cell", Shortcut: "Cmd+M"},
+		{ID: "set-where", Name: "Set Where", Shortcut: ""},
 		{ID: "delete-row", Name: "Delete Row", Shortcut: "Cmd+Backspace"},
 	}
 }
diff --git a/frontend/index.html b/frontend/index.html
index ad7ed40..1050183 100644
--- a/frontend/index.html
+++ b/frontend/index.html
@@ -39,6 +39,24 @@
                 <button id="sort-confirm-btn" class="modal-btn modal-btn-primary">Sort</button>
             </div>
         </div>
+        <div id="set-where-modal" class="hidden">
+            <div class="modal-title">Set Where</div>
+            <div class="modal-body">
+                <label for="sw-match-col">Match Column:</label>
+                <div class="sw-input-wrap">
+                    <input type="text" id="sw-match-col" placeholder="Column name" autocomplete="off" />
+                    <div id="sw-col-autocomplete" class="hidden"></div>
+                </div>
+                <label for="sw-match-values" style="margin-top:10px">Match Values (one per line):</label>
+                <textarea id="sw-match-values" rows="6" placeholder="value1&#10;value2&#10;value3"></textarea>
+                <label for="sw-set-value" style="margin-top:10px">Set Value:</label>
+                <input type="text" id="sw-set-value" placeholder="Value to set" />
+            </div>
+            <div class="modal-actions">
+                <button id="sw-cancel-btn" class="modal-btn">Cancel</button>
+                <button id="sw-confirm-btn" class="modal-btn modal-btn-primary">Apply</button>
+            </div>
+        </div>
         <div id="overlay" class="hidden"></div>
     </div>
     <script src="./src/main.js" type="module"></script>
diff --git a/frontend/src/main.js b/frontend/src/main.js
index 8e3e3b1..38d0bb8 100644
--- a/frontend/src/main.js
+++ b/frontend/src/main.js
@@ -32,6 +32,13 @@ const sortInput = $('#sort-columns-input');
 const sortAutocomplete = $('#sort-autocomplete');
 const sortConfirmBtn = $('#sort-confirm-btn');
 const sortCancelBtn = $('#sort-cancel-btn');
+const swModal = $('#set-where-modal');
+const swMatchCol = $('#sw-match-col');
+const swColAutocomplete = $('#sw-col-autocomplete');
+const swMatchValues = $('#sw-match-values');
+const swSetValue = $('#sw-set-value');
+const swConfirmBtn = $('#sw-confirm-btn');
+const swCancelBtn = $('#sw-cancel-btn');
 
 // Default column width
 const DEFAULT_COL_WIDTH = 120;
@@ -669,6 +676,7 @@ async function loadCommands() {
             { ID: 'sort-desc', Name: 'Sort Z-A', Shortcut: '' },
             { ID: 'sort-advanced', Name: 'Sort Advanced', Shortcut: '' },
             { ID: 'match-cell', Name: 'Match Cell', Shortcut: 'Cmd+M' },
+            { ID: 'set-where', Name: 'Set Where', Shortcut: '' },
             { ID: 'delete-row', Name: 'Delete Row', Shortcut: 'Cmd+Backspace' },
         ];
     }
@@ -755,6 +763,7 @@ commandInput.addEventListener('keydown', (e) => {
 overlay.addEventListener('click', () => {
     if (!commandPalette.classList.contains('hidden')) closeCommandPalette();
     if (!sortModal.classList.contains('hidden')) closeSortAdvanced();
+    if (!swModal.classList.contains('hidden')) closeSetWhere();
 });
 
 // ===== Command execution =====
@@ -776,6 +785,7 @@ async function executeCommand(id) {
         case 'sort-desc': doSort(false); break;
         case 'sort-advanced': openSortAdvanced(); break;
         case 'match-cell': doMatchCell(); break;
+        case 'set-where': openSetWhere(); break;
         case 'delete-row': doDeleteRow(); break;
     }
 }
@@ -1235,6 +1245,209 @@ sortInput.addEventListener('keydown', (e) => {
 sortConfirmBtn.addEventListener('click', confirmSortAdvanced);
 sortCancelBtn.addEventListener('click', closeSortAdvanced);
 
+// ===== Set Where Modal =====
+let swAcIndex = -1;
+
+function openSetWhere() {
+    swModal.classList.remove('hidden');
+    overlay.classList.remove('hidden');
+    swMatchCol.value = '';
+    swMatchValues.value = '';
+    swSetValue.value = '';
+    hideSwAutocomplete();
+    swMatchCol.focus();
+}
+
+function closeSetWhere() {
+    swModal.classList.add('hidden');
+    overlay.classList.add('hidden');
+    swMatchCol.value = '';
+    swMatchValues.value = '';
+    swSetValue.value = '';
+    hideSwAutocomplete();
+}
+
+function hideSwAutocomplete() {
+    swColAutocomplete.classList.add('hidden');
+    swColAutocomplete.innerHTML = '';
+    swAcIndex = -1;
+}
+
+function updateSwAutocomplete() {
+    const text = swMatchCol.value.trim().toLowerCase();
+    const matches = state.headers
+        .filter(h => h.toLowerCase().includes(text))
+        .sort((a, b) => a.toLowerCase().localeCompare(b.toLowerCase()));
+
+    if (matches.length === 0 || (matches.length === 1 && matches[0].toLowerCase() === text)) {
+        hideSwAutocomplete();
+        return;
+    }
+
+    swColAutocomplete.innerHTML = '';
+    swColAutocomplete.classList.remove('hidden');
+    swAcIndex = -1;
+
+    matches.forEach((name) => {
+        const div = document.createElement('div');
+        div.className = 'ac-item';
+        div.textContent = name;
+        div.addEventListener('mousedown', (e) => {
+            e.preventDefault();
+            swMatchCol.value = name;
+            hideSwAutocomplete();
+            swMatchValues.focus();
+        });
+        swColAutocomplete.appendChild(div);
+    });
+}
+
+function confirmSetWhere() {
+    const colName = swMatchCol.value.trim();
+    const matchColIdx = state.headers.findIndex(
+        h => h.toLowerCase() === colName.toLowerCase()
+    );
+    if (matchColIdx === -1) {
+        setStatus(`Unknown column: "${colName}"`);
+        return;
+    }
+
+    const matchVals = new Set(
+        swMatchValues.value.split('\n').map(s => s.trim()).filter(s => s.length > 0)
+    );
+    if (matchVals.size === 0) {
+        setStatus('No match values specified');
+        return;
+    }
+
+    const setValue = swSetValue.value;
+    const targetCol = state.cursor.col;
+    let count = 0;
+
+    for (let r = 0; r < state.rows.length; r++) {
+        const cellVal = state.rows[r][matchColIdx] || '';
+        if (matchVals.has(cellVal)) {
+            setCellValue(r, targetCol, setValue);
+            count++;
+        }
+    }
+
+    closeSetWhere();
+    render();
+    const targetColName = state.headers[targetCol] || colLabel(targetCol);
+    setStatus(`Set ${count} cell(s) in ${targetColName}`);
+}
+
+swMatchCol.addEventListener('input', updateSwAutocomplete);
+swMatchCol.addEventListener('focus', updateSwAutocomplete);
+swMatchCol.addEventListener('blur', () => {
+    setTimeout(hideSwAutocomplete, 150);
+});
+
+swMatchCol.addEventListener('keydown', (e) => {
+    const items = swColAutocomplete.querySelectorAll('.ac-item');
+
+    if (e.key === 'Escape') {
+        e.preventDefault();
+        e.stopPropagation();
+        if (!swColAutocomplete.classList.contains('hidden')) {
+            hideSwAutocomplete();
+        } else {
+            closeSetWhere();
+        }
+        return;
+    }
+
+    if (e.key === 'Enter') {
+        e.preventDefault();
+        e.stopPropagation();
+        if (swAcIndex >= 0 && items[swAcIndex]) {
+            swMatchCol.value = items[swAcIndex].textContent;
+            hideSwAutocomplete();
+            swMatchValues.focus();
+        } else {
+            // Move focus to match values
+            swMatchValues.focus();
+        }
+        return;
+    }
+
+    if (e.key === 'ArrowDown' && items.length > 0) {
+        e.preventDefault();
+        if (swAcIndex >= 0) items[swAcIndex]?.classList.remove('active');
+        swAcIndex = (swAcIndex + 1) % items.length;
+        items[swAcIndex]?.classList.add('active');
+        items[swAcIndex]?.scrollIntoView({ block: 'nearest' });
+        return;
+    }
+
+    if (e.key === 'ArrowUp' && items.length > 0) {
+        e.preventDefault();
+        if (swAcIndex >= 0) items[swAcIndex]?.classList.remove('active');
+        swAcIndex = (swAcIndex - 1 + items.length) % items.length;
+        items[swAcIndex]?.classList.add('active');
+        items[swAcIndex]?.scrollIntoView({ block: 'nearest' });
+        return;
+    }
+
+    if (e.key === 'Tab') {
+        e.preventDefault();
+        if (swAcIndex >= 0 && items[swAcIndex]) {
+            swMatchCol.value = items[swAcIndex].textContent;
+            hideSwAutocomplete();
+        }
+        swMatchValues.focus();
+        return;
+    }
+
+    e.stopPropagation();
+});
+
+swMatchValues.addEventListener('keydown', (e) => {
+    if (e.key === 'Escape') {
+        e.preventDefault();
+        e.stopPropagation();
+        closeSetWhere();
+        return;
+    }
+    // Tab to next field
+    if (e.key === 'Tab' && !e.shiftKey) {
+        e.preventDefault();
+        swSetValue.focus();
+        return;
+    }
+    if (e.key === 'Tab' && e.shiftKey) {
+        e.preventDefault();
+        swMatchCol.focus();
+        return;
+    }
+    e.stopPropagation();
+});
+
+swSetValue.addEventListener('keydown', (e) => {
+    if (e.key === 'Escape') {
+        e.preventDefault();
+        e.stopPropagation();
+        closeSetWhere();
+        return;
+    }
+    if (e.key === 'Enter') {
+        e.preventDefault();
+        e.stopPropagation();
+        confirmSetWhere();
+        return;
+    }
+    if (e.key === 'Tab' && e.shiftKey) {
+        e.preventDefault();
+        swMatchValues.focus();
+        return;
+    }
+    e.stopPropagation();
+});
+
+swConfirmBtn.addEventListener('click', confirmSetWhere);
+swCancelBtn.addEventListener('click', closeSetWhere);
+
 // ===== File loading =====
 async function loadFile(filePath) {
     try {
diff --git a/frontend/src/style.css b/frontend/src/style.css
index ea72bad..1e4eeda 100644
--- a/frontend/src/style.css
+++ b/frontend/src/style.css
@@ -391,6 +391,97 @@ td.cursor-cell {
     background: #1a54a9;
 }
 
+/* Set Where modal */
+#set-where-modal {
+    position: fixed;
+    top: 80px;
+    left: 50%;
+    transform: translateX(-50%);
+    width: 460px;
+    background: #fff;
+    border: 1px solid var(--border);
+    border-radius: 8px;
+    box-shadow: 0 8px 32px rgba(0,0,0,0.2);
+    z-index: 100;
+    overflow: visible;
+}
+
+#set-where-modal.hidden {
+    display: none;
+}
+
+.sw-input-wrap {
+    position: relative;
+}
+
+#sw-match-col {
+    width: 100%;
+    padding: 8px 10px;
+    font-size: 14px;
+    font-family: inherit;
+    border: 1px solid var(--border);
+    border-radius: 4px;
+    outline: none;
+    box-sizing: border-box;
+}
+
+#sw-match-col:focus {
+    border-color: var(--selected-border);
+    box-shadow: 0 0 0 2px rgba(34,102,204,0.15);
+}
+
+#sw-col-autocomplete {
+    position: absolute;
+    left: 0;
+    right: 0;
+    top: 100%;
+    margin-top: 2px;
+    background: #fff;
+    border: 1px solid var(--border);
+    border-radius: 4px;
+    box-shadow: 0 4px 12px rgba(0,0,0,0.12);
+    max-height: 180px;
+    overflow-y: auto;
+    z-index: 110;
+}
+
+#sw-col-autocomplete.hidden {
+    display: none;
+}
+
+#sw-match-values {
+    width: 100%;
+    padding: 8px 10px;
+    font-size: 14px;
+    font-family: inherit;
+    border: 1px solid var(--border);
+    border-radius: 4px;
+    outline: none;
+    resize: vertical;
+    box-sizing: border-box;
+}
+
+#sw-match-values:focus {
+    border-color: var(--selected-border);
+    box-shadow: 0 0 0 2px rgba(34,102,204,0.15);
+}
+
+#sw-set-value {
+    width: 100%;
+    padding: 8px 10px;
+    font-size: 14px;
+    font-family: inherit;
+    border: 1px solid var(--border);
+    border-radius: 4px;
+    outline: none;
+    box-sizing: border-box;
+}
+
+#sw-set-value:focus {
+    border-color: var(--selected-border);
+    box-shadow: 0 0 0 2px rgba(34,102,204,0.15);
+}
+
 /* Drag over style */
 #table-container.drag-over {
     background: var(--selected-bg);