According to most style guides, the words fortunate and fortuitous have different meanings, and yet many people treat them as synonyms. 

The primary meaning of the adjective fortunate is lucky, auspicious, or having good fortune. (The adverb form is fortunately.)

The primary meaning of the adjective fortuitous is accidental or coincidental. (The adverb form is fortuitously.) In contrast to fortunate, fortuitous usually doesn't refer to people.

However, fortuitous is sometimes used interchangeably with fortunate and felicitous. See the usage notes below.

Examples

• "Jane Austen . . . must have been one of those fortunate young women who can enjoy glamour without having illusions."
  (Elizabeth Bowen, "Jane Austen." People, Places, Things, 2008)
• "It is worth noting--would you not agree?--that our lives are often sculpted by the absurd, the unlikely, the purely fortuitous."
  (Tim O'Brien, Tomcat in Love, 1998) 
  
  The best haiku verses have an air of the spontaneous, the fortuitous moment observed.

Usage Notes

• "Although some, seeking pomposity, substitute fortuitous for fortunate, the words are not synonymous. Fortunate means 'lucky.' Fortuitous means 'by chance.' 'by accident.' Something that is fortuitous can also be fortunate, but unless it happened by chance, fortunate is the correct word. It was fortunate that the plane had enough fuel to reach an alternate landing field. The pilot's choice was fortuitous; all the other fields were damaged."
  (Rene J. Cappon, The Associated Press Guide to Writing, Peterson's, 2000)

• "In present-day English we have three senses of fortuitous forming a gradation: 'happening by chance,' 'happening by a lucky chance,' and 'lucky, fortunate.' The third of these has been in use for almost seventy years and is recognized in several dictionaries. There is no question that it is established, especially in newspaper and magazine use, and even though it has lately received a great deal of unfavorable notice, it is showing no sign of going away. You can use the sense, but you should be prepared to catch a little flak if you do.
  
  "It is harder to advise you about the intermediate use. It seems likely to continue in use, and because the element of chance is present in its meaning, it is unlikely to cause much stir. Only one commentator has noticed it so far, and it has not yet been recognized in most dictionaries. Our guess is that if you use fortuitous to mean 'happening by a lucky chance,' you have nothing to worry about."
  (Merriam-Webster's Concise Dictionary of English Usage, Merriam-Webster, 2002)

Practice

(a) "Let's picture a classroom where the more _____ students know at least 90 percent of the words being used by a teacher or in a textbook. That enables them to gain knowledge about the other 10 percent of words that they did not already know."
(E. D. Hirsch, The Making of Americans: Democracy and Our Schools, 2009)

(b) This hour-long documentary is the story of a single photograph: a fascinating tale about a _____ moment frozen in time.

 

Answers to Practice Exercises

Glossary of Usage: Index of Commonly Confused Words