Proliferate (CR 701.27) lets you choose any number of permanents and/or players that already have counters on them, then put one additional counter of each kind that permanent or player already has on each chosen permanent or player.
Planeswalkers enter the battlefield with loyalty counters (CR 306.5b), and loyalty counters are simply a type of counter (CR 122.1). Because loyalty counters are real counters sitting on a permanent, they qualify for proliferate without any special exception.
The key requirement is that the planeswalker must already have at least one loyalty counter on it when you proliferate — a planeswalker reduced to zero loyalty leaves the battlefield immediately (CR 704.5i), so that edge case is not a concern in practice.
Example: You control a Jace, the Mind Sculptor with 3 loyalty counters. You cast Fuel for the Cause, which counters a spell and then proliferates. You choose Jace as one of your proliferate targets; he now has 4 loyalty counters.
Note that adding a loyalty counter via proliferate is not the same as activating a loyalty ability — it does not trigger loyalty-ability restrictions (CR 606.3), and it can push a planeswalker above its printed loyalty value with no problem.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.