IoG chief executive Geoff Webb said: "The synergy between Prospect and the IoG is clear — there is much common ground in terms of both organisations wanting to raise standards, salaries and recognition for their members.
"Sports grounds professionals also now have the opportunity to have the support of professional heavyweight union clout that could make all the difference at the negotiating table."
The move follows recent IoG campaigns to raise the profile of groundsmanship and fight for better pay. It is hoped the backing of the UK's fastest-growing union, with more than 122,000 members, will help groundsmen achieve the recognition and employment rights they deserve.
IoG members will get three months free membership if they sign up before April.