Lifelink causes a player to gain life whenever a source with lifelink deals damage, regardless of who or what is being damaged (CR 702.15a). Fighting is a game action where each creature deals damage equal to its power to the other (CR 701.12). Since fighting generates damage events, lifelink applies normally to those events.
Importantly, a creature does not deal damage to itself in a fight — each creature deals damage to the other creature. So the scenario is better described as: if a creature with lifelink fights another creature, lifelink triggers on the damage it deals to its opponent in the fight.
If both creatures in a fight have lifelink, both controllers gain life equal to their respective creature's power, since both damage events are independent (CR 702.15b covers simultaneous damage with lifelink).
Example: Your 3/3 creature with lifelink fights your opponent's 4/4. Your creature deals 3 damage to the 4/4 — you gain 3 life due to lifelink. The 4/4 deals 4 damage to your 3/3, which dies, and no life is gained (unless the 4/4 also has lifelink).
If a creature somehow dealt damage to itself (e.g., via a unusual effect, not via fight), lifelink would still apply — CR 702.15a sets no restriction on what is being damaged. But standard fight mechanics never cause self-damage.
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.