McClean, who was recently released by Nottingham Forest, was on trial with the Perth outfit in the last few weeks of the 2016-17 campaign.
And the midfielder has now signed a one year deal at McDiarmid Park.
Manager Tommy Wright told the club's website: "I was very impressed with Kyle in his trial and he will go straight into the first team squad."
McClean has been included in Northern Ireland's squad for their opening Euro Under-21 qualifier in Estonia on Thursday.
St Johnstone finished fourth in the Premiership in 2016-17 and go into the Europa League qualifiers later this month.
Find all the latest football transfers on our dedicated page.

In dollar terms, imports dropped 20.4% from a year earlier to $145.2bn, a steeper fall than had been expected.
The drop was due to lower commodity prices and weaker domestic demand.
Next week, China is due to report its third-quarter growth rate, which is expected to be lower than the 7% annual pace seen in the second quarter.
China recently revised down its growth rate for 2014 from 7.4% to 7.3%, the weakest pace for almost 25 years.
China has been attempting to shift from an export-led economy to a consumer-led one, although the steep fall in imports suggests domestic demand is not as strong as the government would have hoped.
In dollar terms, China's exports fell by 3.7% from a year earlier to $205.6bn - although analysts had forecast a steeper fall.
The country's trade surplus nearly doubled to $60.34bn.
In yuan-denominated terms, imports fell by 17.7% while exports were down 1.1%.
In a research note, economists at ANZ said: "September's import figure does not bode well for industrial production and fixed-asset investment.
"Overall growth momentum last month remained weak and third-quarter GDP growth to be released next Monday will likely have edged down to 6.4% in the third quarter, compared with 7% in the first half."

Essex Police said two teenage boys and a man were taken to hospital after being stabbed in Colchester between 18:22 and 18:31 GMT on Saturday.
As a result, the force's temporary deputy chief constable approved use of additional powers to search people.
The "robust policing tactic" was used for three hours to "ensure order had been fully restored", the force said.
A 17-year-old boy and an 18-year-old man from London were arrested, along with a 21-year-old man from Barking.
Ch Insp Simon Anslow said none of those who were stabbed suffered life-threatening injuries.
"This is a rare situation in any town and we are using the full range of legal tools at our disposal to keep the overwhelming majority of people safe, while making sure those intent on causing trouble will be swiftly picked up and dealt with," he said.
"We will be pursuing a vigorous investigation to put all those we can find who are responsible for this detestable behaviour before the courts."
The use of section 60 of the Criminal Justice and Public Order Act 1994 gives officers the power to search people for dangerous objects and weapons without the need to have a reasonable suspicion.
The powers were in force in Colchester town centre from 18:40 until 21:50 GMT on Saturday.
Read more on this and other stories on the BBC Essex Live page
Colchester MP Will Quince said he supported the force's use of the powers in this instance.
"Their swift action in getting the section 60 order, which is a rarely used piece of legislation, means they can stop anybody without reasonable cause to check if they have a weapon within a certain time frame - I applaud the police for doing that," he said.
St Johnstone have made 18-year-old Northern Irish midfielder Kyle McClean their first signing of the summer.
[NEXT_CONCEPT]
China saw a sharp fall in the value of its imports last month, figures show, raising further questions over the strength of its economy.
[NEXT_CONCEPT]
Police invoked special stop and search powers after three people were stabbed within 10 minutes in a town centre.