The National Endowment for the Arts yesterday announced a "special matching grant" to the financially ailing Joffrey Ballet, to help put and keep the popular troupe -- the nation's third largest ballet company -- back on its collective feet.

The grant, which is designed to help the Joffrey meet both immediate and future needs, would give the company up to $250,000 on a 3-to-1 matching basis. fIf maximum matching funds can be secured, the company could realize a maximum of $1 million.

Endowment officials emphasized that the grant is neither a "challenge grant" nor a regular dance program grant, but a special ad hoc award made only after careful review of the company's recent managerial and fund-raising renovations.

Since last January, the company cognizant of its plight, has undergone a wholesale administrative reorganization Henry A. Young Jr., was appointed the first executive director in February, and almost the entire fiscal, development and executive staff is new since then. With the impetus of the new "team," the company has already raised $500,000 toward the required match for the endowment grant, which had been in the planning stages for months.

The Joffrey's problems are well-known. The troupe will not have a New York season this winter or spring, and many of the dancers have had to take jobs with other dance companies, on Broadway, in modeling or television to keep working. Plans for the troupe's first full-length ballet production -- a version of "cinderella" to be choreographed by Robert Joffrey -- have had to be postponed.

During this "transition" year, the company is on 24-week contract, including a 14 1/2-week national tour. The next New York appearances by the troupe will take place in October of 1980.