Ahmadinejad pinned the medal on the commanders military uniform in front of reporters at a press conference in which he also announced that Tehran had decided to release the Britons.
The Revolutionary Guards commander who received the award was identified as Abolqasem Amangah.
At a press conference in London on Saturday, the opposition National Council of Resistance of Iran (NCRI) claimed that Amangah was one of a handful of senior Revolutionary Guards commanders who directed the operation to capture the Britons.
The British sailors were in Iraqi waters at the time of their arrest, the NCRI said, adding that the operation had been premeditated by senior Iranian officials in an effort to win concessions from the West, in particular Britain.
The 15 Britons were seized while conducting a routine early morning anti-smuggling check on a merchant vessel in the Gulf on March 23.