The team owners -- the trusts of Kevin and Edward S. Glazer -- are offering 9.5 million shares.
"The sale is a personal matter for Edward and Kevin,” a spokesman said, according to Bloomberg.
Following the sale, the Glazers will hold a combined 69% stake in the club, down from about 75%.
At last check Manchester United shares were off 12% at $17.22. At that price, the shares are valued at about $164 million.
The club won't receive any proceeds from the sale. Bank of America Securities is the underwriter.
This is the second sale the Glazer family has made this year. In March, Avram Glazer sold a $95 million stake in the company.
Manchester United, one of only a handful of publicly traded sports franchises, jumped earlier this year on speculation that the team would be added to a "Super League" featuring 12 of the most popular football clubs in the world.
Once that deal fell through, the shares declined.
At the same time, the stock received a boost after the team brought back the international superstar Cristiano Ronaldo.
The club is having one of its best starts in years and is near the top of the Premier League rankings after seven matches.