PORTLAND – Maine Coast Heritage Trust has purchased popular Cow Island for $1.5 million and placed conservation restrictions on it to prevent it from being developed.

The statewide land trust announced Monday it has bought the island from private owners and intends to lease it to a youth development organization. The 21-acre island is part of the town of Long Island in Casco Bay, and for years has been a favorite picnic spot for boaters.

The purchase agreement prohibits permanent structures from being built and gives the public continued restricted access.

“This is a terrific day for the people of Maine,” said Jay Espy, president of Maine Coast Heritage Trust.

Cow Island was put on the market for $1.5 million earlier this year by the island’s owners, Hollywood producer Jerry Weintraub and real estate developer and philanthropist Albert Glickman, who lives in Cape Elizabeth.

In May, Rippleffect Inc. secured an option to purchase the island. Rippleffect, which is based in Portland, is a youth development organization for underprivileged young people.

Rippleffect raised more than $400,000 in the past six weeks, which Maine Coast Heritage Trust used along with other funds to acquire the island. With a number of private buyers also interested in the island, the trust was forced to pay the full asking price.

Under its agreement with the land trust, Rippleffect will lease the island and has 30 months to buy it outright. Ted Regan, founder and president of Rippleffect, said his organization would use the property as a classroom, and plans to build caretaker and kayak storage facilities and a seasonal dock.

Cow Island, which is three miles from downtown Portland, is a popular spot for boaters with its views of Hussey Sound, its sand beach and the cement ruins of Fort Lyon, where the military built two gun batteries nearly a century ago.

Fort Lyon was manned for a short time in World War I and was activated again in World War II, when 100 men were stationed there. After the war, the guns were cut up for scrap.