Within Atelidae, they belong to the subfamily Atelinae, alongside other monkeys with prehensile tails like:
* Howler monkeys: Known for their loud, booming calls.
* Woolly monkeys: Have long, thick fur, making them appear fluffy.
* Muriquis: The largest New World monkeys and are known for their peaceful nature.
So, in short, spider monkeys' closest relatives are the other monkeys within the Atelinae subfamily.