The midfielder, who moved to Perth from Dundee United in 2011, was given a short-term deal in the summer intended to allow him to earn an extension for the rest of the campaign.
That had followed his recovery from a broken leg but Robertson then suffered some bone bruising following a collision on the training ground and has just recently returned to full fitness. It has not been quick enough to earn a longer stay in Perth, though.
"Robbo is a great lad and I've no doubt he'll pick up a club," said Tommy Wright, the St Johnstone manager. "But I've had to make the decision based on the circumstances and sentiment can't come into it."