Rankin has left Riccarton after accepting a new job as assistant manager with the cinch Championship side, leaving Naismith in control of the Hearts youth team until the end of the season.
The former Scotland internationalist holds the title of football development manager with the Edinburgh club and had been assisting Rankin with the Under-18s. He will now run the side in the wake of 38-year-old Rankin’s exit.
Hearts sporting director Joe Savage explained that Naismith is an ideal figure to step into the role as he wished Rankin well for the future.
“As much as we would’ve liked to have kept John, he felt the opportunity to take the next step in his coaching journey was something he couldn’t turn down,” Savage told the club’s official website.
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“He goes with our best wishes and thanks for all of his efforts over the past three and a half years.
“We have, in Steven, someone who is vastly experienced and more than capable of leading our Under-18s for the remainder of the campaign.
“He is someone that our young players look up to, such is his playing pedigree, and I’m very confident that he will be able to continue the great work that has been done in developing them for the future.”
Naismith, 35, retired as a player at the end of last season after helping Hearts secure promotion back to the Premiership. He has also been involved in coaching the Scotland national team this season.