The 33-year-old, whose previous clubs also include Derry City and Tranmere Rovers, played 114 times for Rovers.
"It is great to be involved with such a great club as Crusaders - I hope I can bring more success to everyone here on the Shore Road," said McChrystal.
"They have shown over the last lot of years how good a team they are in winning Premierships and other cups."
McChrystal is the latest summer signing for the Crues after the arrival of Brian Jensen, Sean Ward, Jamie Glackin and Mark McAllister.
"I am extremely pleased to get Mark to the club," said Seaview boss Stephen Baxter.
"His pedigree is exactly what we need to join us and he will bring a vast amount of experience as we go into the new season."

About 47% of working South Africans earn less than the wage, which is being introduced to combat income poverty and inequality.
But critics say it could put more people out of work as employers might not be able to afford the higher wages.
The government says it will consult on the issue, but hopes to introduce a minimum wage within two years.
Announcing the rate, which was proposed by a panel of advisers, the country's deputy president Cyril Ramaphosa said: "We are now a step closer to finalising discussions on the national minimum wage. All social partners will now decide what their take is."
He said the panel was not endorsing the proposed figure as a living wage, but wanted to set a minimum payment for workers.
Wages are politically sensitive in the country, where the official unemployment rate is close to 25%.
One employment expert reckoned the figure was only about a quarter of the amount needed for the upkeep of a typical South African working-class household.
Prof Chris Malikane of the University of Witwatersrand told a Johannesburg radio station: "You would need 12,000 rand to sustain a basic household."
The African National Congress, South Africa's ruling party, supported the proposal calling it "credible and clearly supported by clear evidence".
However, the Economic Freedom Fighters, the country's third largest political party, said the plan "favours business at the expense of workers".
It called for a higher minimum wage of at least 4,500 rand.
South Africa faces a possible downgrade to sub-investment grade by credit ratings agencies next month, with concerns remaining over violent wage strikes.
Moody's currently rates South Africa two notches above subinvestment grade, with a negative outlook, while Fitch and S&P Global Ratings have it just a step above "junk".
However, Mr Ramaphosa said: "We have made tremendous progress on the labour instability issues,"
Crusaders have brought in former Bristol Rovers defender Mark McChrystal on a one-year deal.
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South Africa's government has proposed a national minimum wage of 3,500 rand ($242; Â£199) a month.