Templeton made an eye-catching start to the season, quickly establishing himself as an exciting and creative half forward before suffering a broken arm in round six.
Fellow rookie Mav Weller could join the 19-year-old on the senior list in the coming days as the Saints continue a large scale re-signing of their young talent.
The Wash-Up: St Kilda
Despite being sidelined for much of 2014, Templeton said he had a valuable taste of senior football and knew what level he had to reach in 2015.
"It’s been a good learning experience," he told Saints.com.au.
"Obviously the break was pretty disappointing but I’m probably better off long-term for the developments I’ve made off-field during my time off.
"The period on the sidelines honestly went really quickly."
Head of Football Chris Pelchen said Templeton had impressed in training with his endurance, clean hands and commitment and would be an "exciting addition to the Saints primary list".
Coach Alan Richardson said he intended to utilise the teenager's skills and football smarts in the forward line initially.
"He has a very strong athletic profile and is a good decision maker and excellent deliverer of the ball via foot," Richardson told Saints.com.au.
"I expect him to provide another genuine goal-kicking option for us and look forward to watching him grow physically over the summer."
Other Saints to announce recent contract extensions include Luke Dunstan, Jack Newnes, Blake Acres and Spencer White.