diff --git a/crates/pyrefly_config/src/error_kind.rs b/crates/pyrefly_config/src/error_kind.rs
index b26c51a12..b817d3cc7 100644
--- a/crates/pyrefly_config/src/error_kind.rs
+++ b/crates/pyrefly_config/src/error_kind.rs
@@ -131,6 +131,9 @@ pub enum ErrorKind {
     DeleteError,
     /// Calling a function marked with `@deprecated`
     Deprecated,
+    /// Raised when a class implicitly becomes abstract by defining abstract members without
+    /// inheriting from `abc.ABC` or using `abc.ABCMeta`.
+    ImplicitAbstractClass,
     /// This error is raised when Pyrefly infers an implicit `Any`
     ImplicitAny,
     /// Usage of a module that was not actually imported, but does exist.
@@ -276,6 +279,7 @@ impl ErrorKind {
             ErrorKind::Deprecated => Severity::Warn,
             ErrorKind::RedundantCast => Severity::Warn,
             ErrorKind::ImplicitlyDefinedAttribute => Severity::Ignore,
+            ErrorKind::ImplicitAbstractClass => Severity::Ignore,
             ErrorKind::ImplicitAny => Severity::Ignore,
             _ => Severity::Error,
         }
diff --git a/pyrefly/lib/alt/class/class_metadata.rs b/pyrefly/lib/alt/class/class_metadata.rs
index c7de376bb..7aee42c56 100644
--- a/pyrefly/lib/alt/class/class_metadata.rs
+++ b/pyrefly/lib/alt/class/class_metadata.rs
@@ -284,7 +284,7 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         {
             self.validate_frozen_dataclass_inheritance(cls, dm, &bases_with_metadata, errors);
         }
-        let extends_abc = self.extends_abc(&bases_with_metadata);
+        let extends_abc = self.extends_abc(&bases_with_metadata, metaclass.as_ref());
 
         // Compute final base class list.
         let bases = if is_typed_dict && bases_with_metadata.is_empty() {
@@ -971,6 +971,9 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                                 ErrorInfo::Kind(ErrorKind::InvalidArgument),
                                 "Second argument to NewType cannot be a protocol".to_owned(),
                             );
+                            return None;
+                        } else {
+                            return Some((class_object, metadata));
                         }
                     } else if metadata.is_new_type() {
                         self.error(
@@ -1155,7 +1158,11 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
         AbstractClassMembers::new(abstract_members)
     }
 
-    fn extends_abc(&self, bases_with_metadata: &Vec<(Class, Arc<ClassMetadata>)>) -> bool {
+    fn extends_abc(
+        &self,
+        bases_with_metadata: &Vec<(Class, Arc<ClassMetadata>)>,
+        metaclass: Option<&ClassType>,
+    ) -> bool {
         for (base, base_metadata) in bases_with_metadata {
             if base.has_toplevel_qname("abc", "ABC") {
                 return true;
diff --git a/pyrefly/lib/alt/solve.rs b/pyrefly/lib/alt/solve.rs
index cdb565328..43acc9f1f 100644
--- a/pyrefly/lib/alt/solve.rs
+++ b/pyrefly/lib/alt/solve.rs
@@ -318,14 +318,14 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
     ) -> Arc<AbstractClassMembers> {
         let metadata = self.get_metadata_for_class(cls);
         let abstract_members = self.calculate_abstract_members(cls);
-        if metadata.is_final() {
-            let unimplemented = abstract_members.unimplemented_abstract_methods();
-            if !unimplemented.is_empty() {
-                let members = unimplemented
-                    .iter()
-                    .map(|member| format!("`{member}`"))
-                    .collect::<Vec<_>>()
-                    .join(", ");
+        let unimplemented = abstract_members.unimplemented_abstract_methods();
+        if !unimplemented.is_empty() {
+            let members = unimplemented
+                .iter()
+                .map(|member| format!("`{member}`"))
+                .collect::<Vec<_>>()
+                .join(", ");
+            if metadata.is_final() {
                 self.error(
                     errors,
                     cls.range(),
@@ -336,6 +336,24 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
                         members
                     ),
                 );
+            } else if !metadata.is_protocol() && !metadata.is_new_type() {
+                let extends_abc = metadata.extends_abc();
+                let defines_abstract_member = cls.fields().any(|name| {
+                    self.get_field_from_current_class_only(cls, name)
+                        .is_some_and(|field| field.is_abstract())
+                });
+                if !extends_abc && !defines_abstract_member {
+                    self.error(
+                        errors,
+                        cls.range(),
+                        ErrorInfo::Kind(ErrorKind::ImplicitAbstractClass),
+                        format!(
+                            "Class `{}` must implement abstract members: {}",
+                            cls.name(),
+                            members
+                        ),
+                    );
+                }
             }
         }
         Arc::new(abstract_members)
diff --git a/pyrefly/lib/test/abstract_methods.rs b/pyrefly/lib/test/abstract_methods.rs
index 7d5b72021..f6c04ac33 100644
--- a/pyrefly/lib/test/abstract_methods.rs
+++ b/pyrefly/lib/test/abstract_methods.rs
@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+use crate::test::util::TestEnv;
 use crate::testcase;
 
 testcase!(
@@ -85,6 +86,7 @@ drawable = Drawable()  # E: Cannot instantiate `Drawable`
 
 testcase!(
     test_inherited_abstract_method,
+    TestEnv::new().enable_implicit_abstract_class_error(),
     r#"
 from abc import ABC, abstractmethod
 
@@ -93,7 +95,7 @@ class Base(ABC):
     def method(self) -> None:
         pass
 
-class Child(Base):
+class Child(Base):  # E: Class `Child` must implement abstract members: `method`
     # Child doesn't implement method, so it's still abstract
     pass
 
@@ -120,6 +122,7 @@ x = BadClass()  # E: Cannot instantiate `BadClass`
 
 testcase!(
     test_partial_implementation,
+    TestEnv::new().enable_implicit_abstract_class_error(),
     r#"
 from abc import ABC, abstractmethod
 
@@ -132,7 +135,7 @@ class Base(ABC):
     def method2(self) -> None:
         pass
 
-class Partial(Base):
+class Partial(Base):  # E: Class `Partial` must implement abstract members: `method2`
     def method1(self) -> None:
         print("implemented")
 
@@ -205,6 +208,7 @@ c = Child()
 
 testcase!(
     test_abstract_property,
+    TestEnv::new().enable_implicit_abstract_class_error(),
     r#"
 from typing import *
 from abc import ABC, abstractmethod
@@ -215,7 +219,7 @@ class Base(ABC):
     @abstractmethod
     def processor(self) -> bool: pass
 
-class Child(Base):
+class Child(Base):  # E: Class `Child` must implement abstract members: `processor`
     def __init__(self) -> None:
         super().__init__()
 
@@ -225,6 +229,7 @@ x = Child()  # E: Cannot instantiate `Child`
 
 testcase!(
     test_abstract_async_iterator,
+    TestEnv::new().enable_implicit_abstract_class_error(),
     r#"
 from abc import ABC, abstractmethod
 from collections.abc import AsyncIterator
@@ -250,7 +255,7 @@ class C(ABC):
     async def bar(self) -> Any:
         pass
 
-class D(C):
+class D(C):  # E: Class `D` must implement abstract members: `bar`
     async def foo(self) -> AsyncIterator[int]:
         yield 1
     "#,
diff --git a/pyrefly/lib/test/util.rs b/pyrefly/lib/test/util.rs
index a0817a87f..539e25e87 100644
--- a/pyrefly/lib/test/util.rs
+++ b/pyrefly/lib/test/util.rs
@@ -105,6 +105,7 @@ pub struct TestEnv {
     site_package_path: Vec<PathBuf>,
     implicitly_defined_attribute_error: bool,
     implicit_any_error: bool,
+    implicit_abstract_class_error: bool,
     default_require_level: Require,
 }
 
@@ -120,6 +121,7 @@ impl TestEnv {
             site_package_path: Vec::new(),
             implicitly_defined_attribute_error: false,
             implicit_any_error: false,
+            implicit_abstract_class_error: false,
             default_require_level: Require::Exports,
         }
     }
@@ -158,6 +160,11 @@ impl TestEnv {
         self
     }
 
+    pub fn enable_implicit_abstract_class_error(mut self) -> Self {
+        self.implicit_abstract_class_error = true;
+        self
+    }
+
     pub fn with_default_require_level(mut self, level: Require) -> Self {
         self.default_require_level = level;
         self
@@ -231,6 +238,9 @@ impl TestEnv {
         if self.implicit_any_error {
             errors.set_error_severity(ErrorKind::ImplicitAny, Severity::Error);
         }
+        if self.implicit_abstract_class_error {
+            errors.set_error_severity(ErrorKind::ImplicitAbstractClass, Severity::Error);
+        }
         let mut sourcedb = MapDatabase::new(config.get_sys_info());
         for (name, path, _) in self.modules.iter() {
             sourcedb.insert(*name, path.dupe());
diff --git a/website/docs/error-kinds.mdx b/website/docs/error-kinds.mdx
index e03555da6..95fd1ce8d 100644
--- a/website/docs/error-kinds.mdx
+++ b/website/docs/error-kinds.mdx
@@ -347,6 +347,12 @@ def f(): ...
 f()  # deprecated!
 ```
 
+## implicit-abstract-class
+
+Pyrefly emits this error when a class defines abstract members but is not declared abstract (for example, it does not inherit from `abc.ABC` or use `abc.ABCMeta`). Such classes cannot be instantiated because they have unimplemented abstract methods. Add `ABC` as a base class, adjust the metaclass, or provide concrete implementations to resolve the issue.
+
+This error defaults to `Ignore`, so opt in by enabling it in your configuration if you want to enforce explicit abstract base classes.
+
 ## implicit-any
 
 This error is emitted when a Pyrefly infers an implicit `Any` type in your code. This is common in gradually-typed code, but reduces type safety so we provide this error for users that want to enforce fully-typed codebases.