Techniques for lazy copying of runtime-managed stack frames are disclosed. A runtime environment generates a runtime-managed stack including multiple frames. A topmost subset of frames includes data associated with particular instructions and a return address. A lower subset of frames includes data associated with different instructions. The runtime environment stores a copy of the topmost subset of frames in an OS-managed stack, without copying the lower subset. The particular instructions execute using the copy of the topmost subset of frames in the OS-managed stack. The runtime environment replaces, in the copy, the return address with a return barrier address. When execution of the instructions terminates, control passes to return barrier instructions, which store a copy of the lower subset of frames in the OS-managed stack and pass control to the different instructions. The different instructions execute using the copy of the lower subset of frames in the OS-managed stack.