diff --git a/packages/split-layout/src/vaadin-split-layout-mixin.js b/packages/split-layout/src/vaadin-split-layout-mixin.js
index 454025c4c79..9918554a9f5 100644
--- a/packages/split-layout/src/vaadin-split-layout-mixin.js
+++ b/packages/split-layout/src/vaadin-split-layout-mixin.js
@@ -63,17 +63,38 @@ export const SplitLayoutMixin = (superClass) =>
 
     /** @private */
     _processChildren() {
-      [...this.children].forEach((child, i) => {
-        if (i === 0) {
-          this._primaryChild = child;
-          child.setAttribute('slot', 'primary');
-        } else if (i === 1) {
-          this._secondaryChild = child;
-          child.setAttribute('slot', 'secondary');
-        } else {
-          child.removeAttribute('slot');
-        }
-      });
+      const children = [...this.children];
+
+      children.filter((child) => child.hasAttribute('slot')).forEach((child) => this._processChildWithSlot(child));
+
+      children
+        .filter((child) => !child.hasAttribute('slot'))
+        .forEach((child, i) => this._processChildWithoutSlot(child, i));
+    }
+
+    /** @private */
+    _processChildWithSlot(child) {
+      const slot = child.getAttribute('slot');
+      if (child.__autoSlotted) {
+        this[`_${slot}Child`] = null;
+        child.removeAttribute('slot');
+      } else {
+        this[`_${slot}Child`] = child;
+      }
+    }
+
+    /** @private */
+    _processChildWithoutSlot(child, idx) {
+      let slotName;
+      if (this._primaryChild || this._secondaryChild) {
+        slotName = this._primaryChild ? 'secondary' : 'primary';
+      } else {
+        slotName = idx === 0 ? 'primary' : 'secondary';
+      }
+
+      this[`_${slotName}Child`] = child;
+      child.setAttribute('slot', slotName);
+      child.__autoSlotted = true;
     }
 
     /** @private */
diff --git a/packages/split-layout/test/split-layout.common.js b/packages/split-layout/test/split-layout.common.js
index 725b5867af9..4c583510488 100644
--- a/packages/split-layout/test/split-layout.common.js
+++ b/packages/split-layout/test/split-layout.common.js
@@ -73,6 +73,96 @@ describe('split layout', () => {
       expect(getComputedStyle(first).pointerEvents).to.equal('visible');
       expect(getComputedStyle(second).pointerEvents).to.equal('visible');
     });
+
+    describe('elements with slot pre-defined', () => {
+      it('should respect pre-defined slot values in both elements', async () => {
+        const layout = fixtureSync(`
+          <vaadin-split-layout>
+            <div id="second" slot="secondary">secondary</div>
+            <div id="first" slot="primary">primary</div>
+          </vaadin-split-layout>
+        `);
+        await nextRender();
+        expect(layout.querySelector('#first').slot).to.be.equal('primary');
+        expect(layout.querySelector('#second').slot).to.be.equal('secondary');
+      });
+
+      it('should assign a slot if only one element has "secondary" slot pre-defined', async () => {
+        const layout = fixtureSync(`
+          <vaadin-split-layout>
+            <div id="second" slot="secondary">secondary</div>
+            <div id="first">primary</div>
+          </vaadin-split-layout>
+        `);
+        await nextRender();
+        expect(layout.querySelector('#first').slot).to.be.equal('primary');
+        expect(layout.querySelector('#second').slot).to.be.equal('secondary');
+      });
+
+      it('should assign a slot if only element has "primary" slot pre-defined', async () => {
+        const layout = fixtureSync(`
+          <vaadin-split-layout>
+            <div id="second">secondary</div>
+            <div id="first" slot="primary">primary</div>
+          </vaadin-split-layout>
+        `);
+        await nextRender();
+        expect(layout.querySelector('#first').slot).to.be.equal('primary');
+        expect(layout.querySelector('#second').slot).to.be.equal('secondary');
+      });
+
+      it('should respect assigned slot if only one element has slot pre-defined after order is inverted', async () => {
+        const layout = fixtureSync(`
+          <vaadin-split-layout>
+            <div id="second">secondary</div>
+            <div id="first" slot="primary">primary</div>
+          </vaadin-split-layout>
+        `);
+        await nextRender();
+
+        const first = layout.querySelector('#first');
+        layout.prepend(first);
+        await nextRender();
+
+        expect(layout.querySelector('#first').slot).to.be.equal('primary');
+        expect(layout.querySelector('#second').slot).to.be.equal('secondary');
+      });
+
+      it('should swap slots if children without pre-defined slots invert order', async () => {
+        const layout = fixtureSync(`
+          <vaadin-split-layout>
+            <div id="second">secondary</div>
+            <div id="first">primary</div>
+          </vaadin-split-layout>
+        `);
+        await nextRender();
+
+        const second = layout.querySelector('#second');
+        layout.prepend(second);
+        await nextRender();
+
+        expect(layout.querySelector('#first').slot).to.be.equal('secondary');
+        expect(layout.querySelector('#second').slot).to.be.equal('primary');
+      });
+
+      it('should assign slots only for direct children', async () => {
+        const layout = fixtureSync(`
+          <vaadin-split-layout>
+            <div id="first">primary</div>
+            <vaadin-split-layout id="second">
+              <div id="nested-first" slot="primary"></div>
+              <div id="nested-second" slot="secondary"></div>
+            </vaadin-split-layout>
+          </vaadin-split-layout>
+        `);
+        await nextRender();
+
+        const first = layout.querySelector('#first');
+        expect(first.slot).to.be.equal('primary');
+        const second = layout.querySelector('#second');
+        expect(second.slot).to.be.equal('secondary');
+      });
+    });
   });
 });