Jack Marshman beat compatriot John Phillips by split decision in the UFC London 'battle of Wales' at the O2 Arena.
Abertillery's Marshman, 29, scored 28-29, 29-28, 29-28 to take the title and now has three wins from his six fights.
Defeat left Phillips to contemplate the possibility his UFC career is over.
Phillips is trained by Conor McGregor's coach John Kavanagh and, at 31-year-old, he was seeking his first victory on mixed martial arts' top promotion.
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Both Marshman and Phillips had previously lost successive fights in UFC.
Their bout came on the undercard of the Darren Till v Jorge Masvidal showdown.