Speaking to the Sun, Ross Hamilton said that he was just "having a joke".
Describing Tuesday's incident, Hamilton said: "I put my arm round Ant and said, 'Where's your girlfriend?'
"I thought he must be used to that. He didn't think it was funny so I pinched his cheek and said, 'You're cute when you get angry.' He didn't like that, then I just gave him a little slap."
Hamilton said that the pair "exchanged verbals" as McPartlin left, adding: "I'm not sorry. He deserved it."
The altercation happened while McPartlin was at a pub in Chiswick watching the Arsenal versus Barcelona football match.
The presenter apparently said that he was "fine" yesterday while filming for Push the Button.