The President's decision comes as he prepares for talks with North Korean leader Kim Jong-un.
While leaving for California, Trump told reporters that Tillerson "had a different mindset. I think Rex is much happier now".
The President, taking a dig at the dismissed Secretary of State, said Tillerson's replacement Mike Pompeo and he have always been "on the same wavelength".
Trump also praised Pompeo for "tremendous energy, tremendous intellect" and explained Tillerson and I "were not really thinking the same" on issues.
The President said he "really didn't discuss it very much with [Rex], honestly".
"I made that decision by myself," the President said.
The President earlier in the day tweeted: "Mike Pompeo will become our new Secretary of State. He will do fantastic job! Thank you to Rex Tillerson for his service!"
He also said: "Gina Haspel would be the new Director of the CIA, and the first woman so chosen. Congratulations to all!"
The President in a statement also urged Pompeo's "swift confirmation" and said his experience in US military, Congress, and the CIA have prepared him well for his new role.(ANI)