A replaceable RAM slot is there for a reason.

Typically no. If your laptop has a dedicated, user-accessible slot for a replaceable stick of RAM, using it shouldn’t void the warranty. The manufacturer put that slot there expecting you to use it.

The exception is if you physically break something while installing it—snap a clip, crack the motherboard, strip a screw. That damage won’t be covered. But the act of upgrading itself? That’s a normal allowed use.

Some manufacturers (looking at you, certain gaming brands) try to claim any user modification voids the warranty. In the US at least, the Magnuson-Moss Act generally prevents them from voiding the whole warranty just because you opened the case—they have to prove your upgrade caused the failure. Still, keep your original RAM stick just in case you need to send it in for service.

Just be careful, watch a teardown video first, and you’ll be fine.