Strauss made a derogatory remark about Pietersen in the Sky Sports commentary box for their coverage of the MCC v Rest of the World XI match, which Pietersen was playing in, at Lord's on Saturday.
The comment was made during a commercial break, but was picked up by viewers of Fox Sport in Australia, who take Sky's commentary.
An apology from the channel was issued via Sky Cricket's official Twitter account shortly afterwards.
"Earlier comments were made during a break of play which were heard overseas," it read. "We apologise for the language used."
Strauss then issued a brief statement of his own, expressing his deep regret at the situation.
"I apologise unreservedly, particularly to Kevin Pietersen," he said. "I am mortified and profusely sorry."
Strauss and Pietersen were involved in controversy in 2012 when it emerged that Pietersen had sent text messages criticising his then-England captain to the touring South African team.
Pietersen was dropped over the saga and then returned to the national team, but was told in February this year that he was no longer part of England's plans.
Strauss announced his retirement shortly after Pietersen was initially dropped.