diff --git a/frontend/src/styles/test-activity.scss b/frontend/src/styles/test-activity.scss
index 84d852c3b9c6..2bf70c4d44e6 100644
--- a/frontend/src/styles/test-activity.scss
+++ b/frontend/src/styles/test-activity.scss
@@ -115,6 +115,8 @@
   }
 
   .activity {
+    --activity-tooltip-offset: 0px;
+
     grid-area: chart;
     display: grid;
     grid-auto-flow: column;
@@ -131,6 +133,27 @@
       &[data-level="filler"]:hover {
         border: none;
       }
+
+      &[aria-label][data-balloon-pos="up"]::after {
+        font-size: clamp(0.75rem, calc(0.55rem + 1vw), 1rem);
+        line-height: 1.2;
+        max-width: calc(100vw - 1rem);
+        text-align: center;
+        transform: translate(
+          calc(-50% + var(--activity-tooltip-offset)),
+          var(--balloon-move)
+        );
+        white-space: normal;
+        width: max-content;
+      }
+
+      &[aria-label][data-balloon-pos="up"]:hover::after,
+      &[aria-label][data-balloon-pos="up"][data-balloon-visible]::after,
+      &[aria-label][data-balloon-pos="up"]:not(
+          [data-balloon-nofocus]
+        ):focus::after {
+        transform: translate(calc(-50% + var(--activity-tooltip-offset)), 0);
+      }
     }
   }
   .legend {
diff --git a/frontend/src/ts/elements/test-activity.ts b/frontend/src/ts/elements/test-activity.ts
index 7d2bae4b95de..f3e52eaf69c5 100644
--- a/frontend/src/ts/elements/test-activity.ts
+++ b/frontend/src/ts/elements/test-activity.ts
@@ -3,6 +3,74 @@ import {
   TestActivityMonth,
 } from "./test-activity-calendar";
 
+const tooltipViewportPadding = 8;
+
+function getTooltipHorizontalOffset(
+  triggerLeft: number,
+  triggerWidth: number,
+  tooltipWidth: number,
+  viewportWidth: number,
+): number {
+  const availableWidth = Math.max(
+    0,
+    viewportWidth - tooltipViewportPadding * 2,
+  );
+  const renderedTooltipWidth = Math.min(tooltipWidth, availableWidth);
+  const triggerCenter = triggerLeft + triggerWidth / 2;
+  const tooltipLeft = triggerCenter - renderedTooltipWidth / 2;
+  const tooltipRight = triggerCenter + renderedTooltipWidth / 2;
+
+  if (tooltipLeft < tooltipViewportPadding) {
+    return tooltipViewportPadding - tooltipLeft;
+  }
+
+  if (tooltipRight > viewportWidth - tooltipViewportPadding) {
+    return viewportWidth - tooltipViewportPadding - tooltipRight;
+  }
+
+  return 0;
+}
+
+function measureTooltipWidth(element: HTMLElement): number {
+  const label = element.getAttribute("aria-label");
+  if (label === null) {
+    return 0;
+  }
+
+  const tooltipStyle = window.getComputedStyle(element, "::after");
+  const measure = document.createElement("span");
+  measure.textContent = label;
+  Object.assign(measure.style, {
+    fontFamily: tooltipStyle.fontFamily,
+    fontSize: tooltipStyle.fontSize,
+    fontStyle: tooltipStyle.fontStyle,
+    fontWeight: tooltipStyle.fontWeight,
+    letterSpacing: tooltipStyle.letterSpacing,
+    paddingLeft: tooltipStyle.paddingLeft,
+    paddingRight: tooltipStyle.paddingRight,
+    position: "fixed",
+    visibility: "hidden",
+    whiteSpace: "nowrap",
+  });
+  document.body.appendChild(measure);
+  const width = measure.getBoundingClientRect().width;
+  measure.remove();
+
+  return width;
+}
+
+function positionTooltip(element: HTMLElement): void {
+  const triggerRect = element.getBoundingClientRect();
+  const offset = getTooltipHorizontalOffset(
+    triggerRect.left,
+    triggerRect.width,
+    measureTooltipWidth(element),
+    document.documentElement.clientWidth,
+  );
+
+  element.style.setProperty("--activity-tooltip-offset", `${offset}px`);
+}
+
 export function init(
   element: HTMLElement,
   calendar?: TestActivityCalendar,
@@ -57,6 +125,7 @@ export function update(
     if (day.label !== undefined) {
       elem.setAttribute("aria-label", day.label);
       elem.setAttribute("data-balloon-pos", "up");
+      elem.addEventListener("pointerenter", () => positionTooltip(elem));
     }
     container.appendChild(elem);
   }