Fletcher's renown rivalled William Shakespeare's during his lifetime
A spokesman said on Monday the gallery had collected the £218,000 needed to save the piece, the work of an unidentified artist, for the nation.
The National Gallery now has a month to collect all the pledged funds.
Fletcher, born in Rye, East Sussex, was one of the most prolific playwrights of his era.
Sandy Nairne, the director of the National Portrait Gallery, said: "He's looking pretty fine, he's looking not quite pleased with himself, but he's looking terrific and very strong.
"So it happens to be a very good condition and wonderful portrait, and one very important to acquire for the national collection."
In November the gallery received a £50,000 grant from the Art Fund charity towards the portrait.
Fletcher (1579-1625) wrote three of his plays jointly with Shakespeare, The Two Noble Kinsman and Henry VIII which survive, and Cardenio which is now lost.
He also collaborated on numerous works with Francis Beaumont, among them The Maid's Tragedy and Philaster, or Love Lies a-Bleeding.The National Portrait Gallery's collection of 16th and 17th century poets already includes likenesses of Shakespeare, Ben Jonson and John Donne.