Johnstone moved to Colchester on a two-year deal for an undisclosed fee in June 2016
St Johnstone have agreed a season-long loan for Colchester United striker Denny Johnstone.
The 22-year-old joined Saints for training on Thursday morning and is expected to go straight into the squad for Saturday's trip to Celtic Park.
St Johnstone manager Tommy Wright said: "He ticks a lot of boxes for us.
"He has got quality and, in the market we are in, it is a great option. He will improve the team. He is only 22 and he has a presence."
Johnstone came through the ranks at Celtic but did not feature for the first team before joining Birmingham City in 2014.
After loan spells with Macclesfield Town, Cheltenham Town, Burton Albion and Greenock Morton, he signed for Colchester last summer.
He scored twice in 32 appearances last term before his season was cut short after having surgery on a knee injury in February and has made one League Two start in this campaign.
"When you look at his record in the Championship with Morton, he scored 14 or 15 goals in 30 games or so and we want to try to get him back to that," said Wright.
Denny Johnstone (left) scored 17 times in 43 games on loan to Morton
Celtic have not lost to a Scottish side since a 2-1 defeat to St Johnstone at McDiarmid Park on 11 May 2016.
Asked what it would mean to be the one who brought his former club's record to an end, Johnstone said: "It would be massive.
"That would be the best scenario, fingers crossed.
"I had a look at the fixtures and it was a funny one when I saw it and I am delighted that hopefully I will be in the squad for that one.
"I am very much looking forward to going back there.
"Callum McGregor was in the same reserves side as I was and now he is flourishing.
"They are absolutely flying and they still have that record going and all the teams will be desperate to take that away from them."
Source: bbc.com