The competitive rank directly above Diamond in Overwatch is Master. The traditional rank progression is Bronze, Silver, Gold, Platinum, Diamond, Master, and Grandmaster, from lower to higher tiers.
Diamond is therefore immediately below Master, while Grandmaster is the next rank above Master. These names describe a player’s competitive standing rather than a hero role, account level, or ordinary experience level. A player can perform well in quick-play matches without changing their competitive rank.
Overwatch 2 changed how competitive rank updates were displayed over time. At launch, players generally saw a rank update after a set number of wins or losses; later seasons moved toward adjustments after individual matches and introduced additional competitive-system changes. Those presentation changes did not alter the basic ordering of Diamond, Master, and Grandmaster.
Another common mix-up is confusing Master with Grandmaster because both are high-level tiers. In the standard sequence, Master is the single rank immediately above Diamond, making it the canonical answer to this question.