Bradley James - who played Arthur on the BBC fantasy drama - said that he was "beaming with pride" following Morgan's triumph in the category of 'Drama Performance: Male'.
"Don't know if it was obvious by the way I kept hitting him, but was and still am, beaming with pride for NTA 'Best Actor' Colin Morgan," he quipped on Twitter.
Rupert Young - Merlin's Sir Leon - also called Morgan "a legend and a worthy winner", while Alexander Vlahos (Mordred) described his co-star as "magnificent".
"Colin Morgan deserved that so much," wrote Vlahos. "Very fortunate to call him a friend and work alongside him."
Eoin Macken (Sir Gwaine) also tweeted to poke fun at rival Downton Abbey, which beat Merlin in the drama category.
"We didn't have any slow motion cricket - can't compete with that," he teased.
Speaking after the NTAs last night, Colin Morgan claimed that the rumoured Merlin movie is unlikely to happen in the near future.
> Merlin star Colin Morgan joins Gabriel Byrne in BBC drama Quirke
Watch a clip of Colin Morgan in Merlin below:
[Gallery: National Television Awards winners]