diff --git a/webapp/app.js b/webapp/app.js
index 82d9f23..6c9c658 100644
--- a/webapp/app.js
+++ b/webapp/app.js
@@ -268,6 +268,7 @@ const setsLabel = document.getElementById("sets-label");
 const setsList = document.getElementById("sets-list");
 const repsInput = document.getElementById("inp-reps");
 const weightInput = document.getElementById("inp-weight");
+const btnWeightSign = document.getElementById("btn-weight-sign");
 const btnAddSet = document.getElementById("btn-add-set");
 const btnSaveWorkout = document.getElementById("btn-save-workout");
 const workoutExercises = document.getElementById("workout-exercises");
@@ -351,6 +352,7 @@ function startExercise(name) {
   nameInput.value = "";
   repsInput.value = "";
   weightInput.value = "";
+  syncWeightSignUI();
   repsInput.focus();
   notesSection.classList.remove("hidden");
   stopRestTimer();
@@ -420,6 +422,7 @@ function addSet() {
 
   repsInput.value = "";
   weightInput.value = weight ? String(weight) : "";
+  syncWeightSignUI();
   repsInput.focus();
   tg.HapticFeedback.impactOccurred("light");
   saveDraft();
@@ -427,6 +430,30 @@ function addSet() {
 
 btnAddSet.addEventListener("click", addSet);
 
+function syncWeightSignUI() {
+  const v = weightInput.value.trim();
+  const negative = v.startsWith("-");
+  btnWeightSign.classList.toggle("active", negative);
+}
+
+btnWeightSign.addEventListener("click", () => {
+  // Flip the sign of the current weight value. If empty, start with "-"
+  // so the user can type digits after it (iOS numeric keypad has no minus).
+  const v = weightInput.value.trim();
+  if (v === "" || v === "-") {
+    weightInput.value = v === "-" ? "" : "-";
+  } else if (v.startsWith("-")) {
+    weightInput.value = v.slice(1);
+  } else {
+    weightInput.value = "-" + v;
+  }
+  syncWeightSignUI();
+  weightInput.focus();
+  tg.HapticFeedback.selectionChanged();
+});
+
+weightInput.addEventListener("input", syncWeightSignUI);
+
 repsInput.addEventListener("keydown", (e) => {
   if (e.key === "Enter") {
     e.preventDefault();
@@ -561,6 +588,7 @@ function editExercise(idx) {
   // Pre-fill weight input with last set's weight for convenience
   const lastWeight = ex.sets_detail?.length ? ex.sets_detail[ex.sets_detail.length - 1].weight_kg : 0;
   weightInput.value = lastWeight ? String(lastWeight) : "";
+  syncWeightSignUI();
   repsInput.value = "";
   repsInput.focus();
 
diff --git a/webapp/index.html b/webapp/index.html
index 7e8958d..6efa8a4 100644
--- a/webapp/index.html
+++ b/webapp/index.html
@@ -42,6 +42,7 @@
           <input type="text" id="inp-reps" class="input input-small" placeholder="Reps" inputmode="numeric" pattern="[0-9]*" />
           <span class="set-separator">x</span>
           <input type="text" id="inp-weight" class="input input-small" placeholder="kg" inputmode="decimal" />
+          <button id="btn-weight-sign" class="btn-sign" title="Flip sign (for assisted exercises)" type="button">&plusmn;</button>
           <button id="btn-add-set" class="btn-icon" title="Add set">+</button>
         </div>
       </div>
diff --git a/webapp/style.css b/webapp/style.css
index e9a57ab..5e5e144 100644
--- a/webapp/style.css
+++ b/webapp/style.css
@@ -107,6 +107,26 @@ body {
 
 .btn-icon:active { opacity: 0.7; }
 
+.btn-sign {
+  width: 36px;
+  height: 44px;
+  border-radius: 10px;
+  border: 1px solid var(--tg-theme-hint-color, #999)55;
+  background: var(--tg-theme-secondary-bg-color, #f0f0f0);
+  color: var(--tg-theme-text-color, #000);
+  font-size: 18px;
+  cursor: pointer;
+  flex-shrink: 0;
+}
+
+.btn-sign:active { opacity: 0.6; }
+
+.btn-sign.active {
+  background: var(--tg-theme-button-color, #3390ec);
+  color: var(--tg-theme-button-text-color, #fff);
+  border-color: var(--tg-theme-button-color, #3390ec);
+}
+
 .btn-link {
   background: none;
   border: none;