"Both players are greatly talented individuals, who are highly regarded members of the squad.
"Our aim for Ulster has always been to create a competitive team focused on a core of Ulster men."
Speaking about his contract renewal, Humphreys said: "Growing up, Ulster was always the place I wanted to play rugby. To know that I will be here, in my home province, for another few years is brilliant, especially as the team are developing in such a positive way. I believe we have a bright future ahead of us."
After joining Ulster ahead of the 2008/09 season, Humphreys has gained 32 caps to date and has racked up 252 points in the Magners League and Heineken Cup competitions.
From his Ulster senior debut in 2005, Caldwell has earned 69 caps for the province.
Both players have also been named in the Ireland 'A' squad to face England Saxons on Sunday in Bath.