Boro captain Jonny Howson registered his 600th career appearance at Coventry City on Saturday.
The skipper, 33, has made 187 of those in a Boro shirt, and one more for previous club Norwich City. He began his career with boyhood club Leeds United, where he made the remaining 225.
Manager Neil Warnock has regularly spoken of his admiration for the midfielder during his tenure.
"I just think Jonny's got every attribute you need at the top level," he said earlier this year.
"It’s only when you have somebody like him in your camp that you realise just what he gives to the club."
The last player to clock up the mammoth milestone while a Boro player was Stewart Downing, who also went on to reach 400 in a Teessiders shirt.