The 31-year-old joins from Dundee United for an undisclosed fee.
Nicky has been a consistent goalscorer throughout his career at clubs such as Rangers and Dunfermline.
Most recently, he scored 43 goals in 145 games for the Tannadice club.
Nicky was at McDiarmid Park this morning and joined up with Callum Davidson, the backroom staff and his new team-mates.
Clark spoke to SaintsTV about his move and said: "I am delighted to be here, it happened quickly over the last week.
"It has been good today to come in and meet the manager and players. I will be training today with the guys and it will be good to get a feel for the squad.
"As soon as I heard the club was interested I wanted to come here. The club has been successful over the last few years.
"Last year was difficult but it showed how much of a togetherness the squad had to get through such a tough spell.
"In football it helps to feel wanted, I felt that straight away after speaking to the manager. That really attracted me to join the club.
"I can't wait to get going and to help score goals for the club.
"I was a young boy when my dad (Sandy) was in charge here, I have brilliant memories from that time as the club qualified for Europe.
"Me, my brother and my mum used to come here every week. It would be great to help the club get back up toward the top end of the table.
"I don't set personal targets but I want to come in and work hard, hopefully I can score plenty of goals and get us up the table."