Ahmadinejad was approved by supreme leader Ayatollah Ali Khamenei at a ceremony on Monday and is to formally take the oath of office in parliament on Wednesday.
"What we saw yesterday shows the divine legitimacy of the president," Kayhan said in an editorial.
Kayhan has been a harsh critic of Ahmadinejad over his appointment last month of a controversial aide Esfandiar Rahim Mashaie as his first deputy and then his delay in dismissing him despite Khamenei's orders.
The paper's editor-in-chief is appointed by Khamenei, who has the final say on all matters of state in Iran.