diff --git a/src/fn_call.rs b/src/fn_call.rs
index d8794fed469..ae6aff10ac2 100644
--- a/src/fn_call.rs
+++ b/src/fn_call.rs
@@ -147,6 +147,45 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a + 'mir>: crate::MiriEvalContextExt<'
                     )?;
                 }
             }
+            "realloc" => {
+                let old_ptr = this.read_scalar(args[0])?.not_undef()?;
+                let new_size = this.read_scalar(args[1])?.to_usize(this)?;
+                let align = this.tcx.data_layout.pointer_align.abi;
+                if old_ptr.is_null_ptr(this) {
+                    if new_size == 0 {
+                        this.write_null(dest)?;
+                    } else {
+                        let new_ptr = this.memory_mut().allocate(
+                            Size::from_bytes(new_size),
+                            align,
+                            MiriMemoryKind::C.into()
+                        );
+                        this.write_scalar(Scalar::Ptr(new_ptr), dest)?;
+                    }
+                } else {
+                    let old_ptr = old_ptr.to_ptr()?;
+                    let memory = this.memory_mut();
+                    let old_size = Size::from_bytes(memory.get(old_ptr.alloc_id)?.bytes.len() as u64);
+                    if new_size == 0 {
+                        memory.deallocate(
+                            old_ptr,
+                            Some((old_size, align)),
+                            MiriMemoryKind::C.into(),
+                        )?;
+                        this.write_null(dest)?;
+                    } else {
+                        let new_ptr = memory.reallocate(
+                            old_ptr,
+                            old_size,
+                            align,
+                            Size::from_bytes(new_size),
+                            align,
+                            MiriMemoryKind::C.into(),
+                        )?;
+                        this.write_scalar(Scalar::Ptr(new_ptr), dest)?;
+                    }
+                }
+            }
 
             "__rust_alloc" => {
                 let size = this.read_scalar(args[0])?.to_usize(this)?;
diff --git a/tests/run-pass/realloc.rs b/tests/run-pass/realloc.rs
new file mode 100644
index 00000000000..c23b3e645c7
--- /dev/null
+++ b/tests/run-pass/realloc.rs
@@ -0,0 +1,45 @@
+//ignore-windows: Uses POSIX APIs
+
+#![feature(rustc_private)]
+
+use core::{slice, ptr};
+
+extern crate libc;
+
+fn main() {
+    unsafe {
+        // Use calloc for initialized memory
+        let p1 = libc::calloc(20, 1);
+
+        // old size < new size
+        let p2 = libc::realloc(p1, 40);
+        let slice = slice::from_raw_parts(p2 as *const u8, 20);
+        assert_eq!(&slice, &[0_u8; 20]);
+
+        // old size == new size
+        let p3 = libc::realloc(p2, 40);
+        let slice = slice::from_raw_parts(p3 as *const u8, 20);
+        assert_eq!(&slice, &[0_u8; 20]);
+
+        // old size > new size
+        let p4 = libc::realloc(p3, 10);
+        let slice = slice::from_raw_parts(p4 as *const u8, 10);
+        assert_eq!(&slice, &[0_u8; 10]);
+
+        libc::free(p4);
+    }
+
+    unsafe {
+        let p1 = libc::malloc(20);
+
+        let p2 = libc::realloc(p1, 0);
+        assert!(p2.is_null());
+    }
+
+    unsafe {
+        let p1 = libc::realloc(ptr::null_mut(), 20);
+        assert!(!p1.is_null());
+
+        libc::free(p1);
+    }
+}