Winona’s popular riverfront attraction, the Minnesota Marine Art Museum (MMAM), is adding another wing to house its fast-growing collection of 19th century American art.

The $1.9 million addition announced Sunday will house the Richard and Jane Manoogian Gallery, a 4,000-square-foot showcase for masterpieces from the museum’s collection of Hudson River School landscapes. It is the second expansion in as many years, following close on the heels of a $1.1 million gallery that opened last September.

“The museum is already underway on the largest expansion in our young history,” said Andrew Maus, the museum’s executive director.

Construction began in March and is expected to be finished in late September, Maus said, although the museum held off on a formal announcement until “we knew the funding was there.”

The Manoogians, a Detroit couple whose $1 billion fortune derives from building and home-improvement products, are providing $500,000 for the project over five years through their family foundation. The remaining $1.4 million was raised from museum supporters in Winona.

The Manoogians’ involvement happened quickly following Richard Manoogian’s first visit to the museum a few months ago. A champion of American art, Manoogian takes patriotic pride in promoting the nation’s cultural heritage, which he regards as underappreciated. A selection of paintings from his collection was shown at the National Gallery of Art in Washington, D.C., in 1989 followed by museums in San Francisco, New York and Detroit.

“He landed, came to the museum, I welcomed him, and he immediately understood and appreciated what’s going on here,” Maus said of Manoogian’s visit. “He’s a major collector of American art and a big ­ advocate for showing it, so that is well aligned with this museum.”

Winona architect Owen Warneke, who designed the existing MMAM, is planning the new spaces, which are expected to retain the folksy, shingle-sided New England style of the present structure on the Mississippi riverfront. It will be built by Schwab LLC, the Winona contractor that has handled all of the museum’s previous construction.

Until now, development of the MMAM has been primarily a local affair. Founded in 2006, it grew out of the private collection of Robert Kierlin and his wife, Mary Burrichter, who began collecting seascapes 14 year ago when they married and moved into a new house. When their art enthusiasm overwhelmed their walls, they launched the MMAM with the help of other Winonans. The Kierlin-Burrichters remain the museum’s ­primary benefactors, and much of their collection of Impressionist, American and marine art is on long-term loan to the museum.

The museum also unveiled five new acquisitions Sunday: a 1941 Maine seascape by the American modernist John Marin and four large pictures by members of the Hudson River School: William Mason Brown, James Hart, James Hope and Richard Hubbard.

With an annual budget of just $650,000 and a full-time staff of five, the museum maintains an ambitious exhibition schedule of about 10 temporary shows each year and has attracted visitors from 20 countries and all 50 states.