diff --git a/.changeset/allow-undefined-classnames.md b/.changeset/allow-undefined-classnames.md
new file mode 100644
index 0000000000..03990473f8
--- /dev/null
+++ b/.changeset/allow-undefined-classnames.md
@@ -0,0 +1,6 @@
+---
+"react-day-picker": patch
+"@daypicker/react": patch
+---
+
+fix: allow `undefined` in `ClassNames` values so `classNames` type-checks under `exactOptionalPropertyTypes`.
diff --git a/packages/react-day-picker/src/helpers/getClassNamesForModifiers.ts b/packages/react-day-picker/src/helpers/getClassNamesForModifiers.ts
index 572f1b3dea..887a2fb5f1 100644
--- a/packages/react-day-picker/src/helpers/getClassNamesForModifiers.ts
+++ b/packages/react-day-picker/src/helpers/getClassNamesForModifiers.ts
@@ -24,10 +24,17 @@ export function getClassNamesForModifiers(
       (previousValue, [key]) => {
         if (modifiersClassNames[key]) {
           previousValue.push(modifiersClassNames[key as string]);
-        } else if (classNames[DayFlag[key as DayFlag]]) {
-          previousValue.push(classNames[DayFlag[key as DayFlag]]);
-        } else if (classNames[SelectionState[key as SelectionState]]) {
-          previousValue.push(classNames[SelectionState[key as SelectionState]]);
+        } else {
+          const dayFlagClassName = classNames[DayFlag[key as DayFlag]];
+          if (dayFlagClassName) {
+            previousValue.push(dayFlagClassName);
+          } else {
+            const selectionClassName =
+              classNames[SelectionState[key as SelectionState]];
+            if (selectionClassName) {
+              previousValue.push(selectionClassName);
+            }
+          }
         }
         return previousValue;
       },
diff --git a/packages/react-day-picker/src/types/props.test.tsx b/packages/react-day-picker/src/types/props.test.tsx
index 51f171a840..79df4c518a 100644
--- a/packages/react-day-picker/src/types/props.test.tsx
+++ b/packages/react-day-picker/src/types/props.test.tsx
@@ -5,7 +5,7 @@ import { getDefaultClassNames } from "../helpers/getDefaultClassNames";
 import type { useDayPicker } from "../useDayPicker";
 
 import type { DayPickerProps } from "./props";
-import type { DateRange, Matcher } from "./shared";
+import type { ClassNames, DateRange, Matcher } from "./shared";
 
 // Keep public class name outputs assignable to string for existing consumers.
 void (getDefaultClassNames().root satisfies string);
@@ -27,6 +27,12 @@ type DateLike = {
   getDate(): number;
 };
 
+// ClassNames values must include `undefined` so `Partial<ClassNames>` accepts
+// `string | undefined` under `exactOptionalPropertyTypes` (issue #3015).
+void (true satisfies undefined extends ClassNames[keyof ClassNames]
+  ? true
+  : false);
+
 const date = new Date(2024, 0, 15);
 const month = new Date(2024, 0, 1);
 const endMonth = new Date(2024, 11, 1);
diff --git a/packages/react-day-picker/src/types/shared.ts b/packages/react-day-picker/src/types/shared.ts
index 0da93d6d8b..e289ad8993 100644
--- a/packages/react-day-picker/src/types/shared.ts
+++ b/packages/react-day-picker/src/types/shared.ts
@@ -251,7 +251,7 @@ export type MonthChangeEventHandler = (month: Date) => void;
  *   };
  */
 export type ClassNames = {
-  [key in UI | SelectionState | DayFlag | Animation]: string;
+  [key in UI | SelectionState | DayFlag | Animation]: string | undefined;
 };
 
 /**
diff --git a/packages/react-day-picker/src/useAnimation.ts b/packages/react-day-picker/src/useAnimation.ts
index 81a88a5079..cd9c76a7a0 100644
--- a/packages/react-day-picker/src/useAnimation.ts
+++ b/packages/react-day-picker/src/useAnimation.ts
@@ -115,12 +115,12 @@ export function useAnimation(
 
         // remove animation classes from the new month snapshots
         const captionEl = queryCaptionEl(currentMonthElSnapshot);
-        if (captionEl) {
+        if (captionEl && captionAnimationClass) {
           captionEl.classList.remove(captionAnimationClass);
         }
 
         const weeksEl = queryWeeksEl(currentMonthElSnapshot);
-        if (weeksEl) {
+        if (weeksEl && weeksAnimationClass) {
           weeksEl.classList.remove(weeksAnimationClass);
         }
       });
@@ -172,12 +172,12 @@ export function useAnimation(
         currentMonthEl.style.position = "relative";
         currentMonthEl.style.overflow = "hidden";
         const captionEl = queryCaptionEl(currentMonthEl);
-        if (captionEl) {
+        if (captionEl && captionAnimationClass) {
           captionEl.classList.add(captionAnimationClass);
         }
 
         const weeksEl = queryWeeksEl(currentMonthEl);
-        if (weeksEl) {
+        if (weeksEl && weeksAnimationClass) {
           weeksEl.classList.add(weeksAnimationClass);
         }
         // animate new displayed month end
@@ -192,10 +192,10 @@ export function useAnimation(
             navEl.style.zIndex = "";
           }
 
-          if (captionEl) {
+          if (captionEl && captionAnimationClass) {
             captionEl.classList.remove(captionAnimationClass);
           }
-          if (weeksEl) {
+          if (weeksEl && weeksAnimationClass) {
             weeksEl.classList.remove(weeksAnimationClass);
           }
           currentMonthEl.style.position = "";
@@ -220,21 +220,23 @@ export function useAnimation(
 
         const previousCaptionEl = queryCaptionEl(previousMonthEl);
         if (previousCaptionEl) {
-          previousCaptionEl.classList.add(
-            isAfterPreviousMonth
-              ? classNames[Animation.caption_before_exit]
-              : classNames[Animation.caption_after_exit],
-          );
+          const captionExitClass = isAfterPreviousMonth
+            ? classNames[Animation.caption_before_exit]
+            : classNames[Animation.caption_after_exit];
+          if (captionExitClass) {
+            previousCaptionEl.classList.add(captionExitClass);
+          }
           previousCaptionEl.addEventListener("animationend", cleanUp);
         }
 
         const previousWeeksEl = queryWeeksEl(previousMonthEl);
         if (previousWeeksEl) {
-          previousWeeksEl.classList.add(
-            isAfterPreviousMonth
-              ? classNames[Animation.weeks_before_exit]
-              : classNames[Animation.weeks_after_exit],
-          );
+          const weeksExitClass = isAfterPreviousMonth
+            ? classNames[Animation.weeks_before_exit]
+            : classNames[Animation.weeks_after_exit];
+          if (weeksExitClass) {
+            previousWeeksEl.classList.add(weeksExitClass);
+          }
         }
 
         currentMonthEl.insertBefore(previousMonthEl, currentMonthEl.firstChild);