diff --git a/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java b/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java
index 91e815e73..f12dcb8e8 100644
--- a/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java
+++ b/pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java
@@ -1441,6 +1441,8 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
     var scope = (ModuleScope) symbolTable.getCurrentScope();
     scope.setModifiers(modifiers);
 
+    checkAbstractMembersAllowed(modifiers, mod.getProperties(), mod.getMethods());
+
     // visit imports first so that we already have the object member name available
     var imports = mod.getImports();
     var importMembers = new ObjectMember[imports.size()];
@@ -1685,6 +1687,7 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
           List<ClassProperty> properties = bodyNode != null ? bodyNode.getProperties() : List.of();
           List<ClassMethod> methods = bodyNode != null ? bodyNode.getMethods() : List.of();
           registerClassScopeNames(scope, properties, methods);
+          checkAbstractMembersAllowed(modifiers, properties, methods);
 
           var supertypeCtx = clazz.getSuperClass();
 
@@ -1775,6 +1778,30 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
     };
   }
 
+  private void checkAbstractMembersAllowed(
+      int enclosingModifiers, List<ClassProperty> properties, List<ClassMethod> methods) {
+    if (VmModifier.isAbstract(enclosingModifiers)) {
+      return;
+    }
+    for (var property : properties) {
+      checkMemberNotAbstract(property.getModifiers());
+    }
+    for (var method : methods) {
+      checkMemberNotAbstract(method.getModifiers());
+    }
+  }
+
+  private void checkMemberNotAbstract(List<Modifier> modifiers) {
+    for (var modifier : modifiers) {
+      if (modifier.getValue() == ModifierValue.ABSTRACT) {
+        throw exceptionBuilder()
+            .evalError("abstractMemberInNonAbstractClass")
+            .withSourceSection(createSourceSection(modifier.span()))
+            .build();
+      }
+    }
+  }
+
   private UnresolvedPropertyNode[] doVisitClassProperties(
       List<ClassProperty> propertyContexts, Set<String> propertyNames) {
     var propertyNodes = new UnresolvedPropertyNode[propertyContexts.size()];
diff --git a/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties b/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties
index 0f378bb18..7ad9603aa 100644
--- a/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties
+++ b/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties
@@ -286,6 +286,11 @@ External members cannot have a body.
 abstractMemberCannotHaveBody=\
 Abstract members cannot have a body.
 
+abstractMemberInNonAbstractClass=\
+Cannot define an abstract member in a non-abstract class.\n\
+\n\
+A member can only be `abstract` if its enclosing class is also `abstract`.
+
 methodNotDefined1=\
 Method `{0}` is not defined for argument type `{1}`.
 
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractClass.pkl b/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractClass.pkl
new file mode 100644
index 000000000..7417fbd26
--- /dev/null
+++ b/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractClass.pkl
@@ -0,0 +1,5 @@
+class Foo {
+  abstract bar: Int
+}
+
+res = new Foo { bar = 5 }
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractModule.pkl b/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractModule.pkl
new file mode 100644
index 000000000..e9dff5a3d
--- /dev/null
+++ b/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractModule.pkl
@@ -0,0 +1 @@
+abstract foo: Int
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMethodInNonAbstractClass.pkl b/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMethodInNonAbstractClass.pkl
new file mode 100644
index 000000000..5f2a1213b
--- /dev/null
+++ b/pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMethodInNonAbstractClass.pkl
@@ -0,0 +1,5 @@
+class Foo {
+  abstract function bar(): Int
+}
+
+res = new Foo {}
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractClass.err b/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractClass.err
new file mode 100644
index 000000000..836ca8f00
--- /dev/null
+++ b/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractClass.err
@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Cannot define an abstract member in a non-abstract class.
+
+x | abstract bar: Int
+    ^^^^^^^^
+at abstractMemberInNonAbstractClass#Foo (file:///$snippetsDir/input/errors/abstractMemberInNonAbstractClass.pkl)
+
+A member can only be `abstract` if its enclosing class is also `abstract`.
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractModule.err b/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractModule.err
new file mode 100644
index 000000000..d31c181d0
--- /dev/null
+++ b/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractModule.err
@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Cannot define an abstract member in a non-abstract class.
+
+x | abstract foo: Int
+    ^^^^^^^^
+at abstractMemberInNonAbstractModule (file:///$snippetsDir/input/errors/abstractMemberInNonAbstractModule.pkl)
+
+A member can only be `abstract` if its enclosing class is also `abstract`.
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMethodInNonAbstractClass.err b/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMethodInNonAbstractClass.err
new file mode 100644
index 000000000..a090b927a
--- /dev/null
+++ b/pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMethodInNonAbstractClass.err
@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Cannot define an abstract member in a non-abstract class.
+
+x | abstract function bar(): Int
+    ^^^^^^^^
+at abstractMethodInNonAbstractClass#Foo (file:///$snippetsDir/input/errors/abstractMethodInNonAbstractClass.pkl)
+
+A member can only be `abstract` if its enclosing class is also `abstract`.
diff --git a/pkl-doc/src/test/files/DocGeneratorTest/input/com.package1/moduleMethodModifiers.pkl b/pkl-doc/src/test/files/DocGeneratorTest/input/com.package1/moduleMethodModifiers.pkl
index 6138dbb76..bd48b43c7 100644
--- a/pkl-doc/src/test/files/DocGeneratorTest/input/com.package1/moduleMethodModifiers.pkl
+++ b/pkl-doc/src/test/files/DocGeneratorTest/input/com.package1/moduleMethodModifiers.pkl
@@ -1,5 +1,5 @@
 /// Module methods with different modifiers.
-module com.package1.moduleMethodModifiers
+abstract module com.package1.moduleMethodModifiers
 
 /// Method with `abstract` modifier.
 abstract function method1(arg: String): Boolean
diff --git a/pkl-doc/src/test/files/DocGeneratorTest/output/run-1/com.package1/1.2.3/moduleMethodModifiers/index.html b/pkl-doc/src/test/files/DocGeneratorTest/output/run-1/com.package1/1.2.3/moduleMethodModifiers/index.html
index 573620801..06c96ad88 100644
--- a/pkl-doc/src/test/files/DocGeneratorTest/output/run-1/com.package1/1.2.3/moduleMethodModifiers/index.html
+++ b/pkl-doc/src/test/files/DocGeneratorTest/output/run-1/com.package1/1.2.3/moduleMethodModifiers/index.html
@@ -32,7 +32,7 @@
       </ul>
       <div id="_overview" class="anchor"> </div>
       <div id="_declaration" class="member">
-        <div class="member-signature">module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
+        <div class="member-signature">abstract module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
         <div class="doc-comment"><p>Module methods with different modifiers.</p></div>
         <dl class="member-info">
           <dt class="">Module URI:</dt>
diff --git a/pkl-doc/src/test/files/DocGeneratorTest/output/run-2/com.package1/1.2.3/moduleMethodModifiers/index.html b/pkl-doc/src/test/files/DocGeneratorTest/output/run-2/com.package1/1.2.3/moduleMethodModifiers/index.html
index 573620801..06c96ad88 100644
--- a/pkl-doc/src/test/files/DocGeneratorTest/output/run-2/com.package1/1.2.3/moduleMethodModifiers/index.html
+++ b/pkl-doc/src/test/files/DocGeneratorTest/output/run-2/com.package1/1.2.3/moduleMethodModifiers/index.html
@@ -32,7 +32,7 @@
       </ul>
       <div id="_overview" class="anchor"> </div>
       <div id="_declaration" class="member">
-        <div class="member-signature">module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
+        <div class="member-signature">abstract module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
         <div class="doc-comment"><p>Module methods with different modifiers.</p></div>
         <dl class="member-info">
           <dt class="">Module URI:</dt>