The two have been a couple since William's university days, and speculation about their future is a continuing subject of gossip and speculation.
With grandmother Queen Elizabeth II addressing the new second lieutenants and father Prince Charles taking the salute, William was one face in a crowd of cadets participating in "passing out" ceremonies at the Royal Military Academy, Sandhurst, following a 44-week commissioning course.
William's younger brother, Prince Harry, was commissioned at a similar ceremony earlier this year. Both of the young royals were assigned to the Blues and Royals regiment of the Household Cavalry.
Middleton, wearing a bright scarlet coat and dark hat, attracted lingering gazes from the television cameras covering the ceremony.
"For those who are to be commissioned today and to those who will shortly follow, a great deal will be expected of you," the queen told cadets.
"You must be courageous yet selfless, leaders yet carers, confident yet considerate... These are very special attributes, but those whom you will command and your country too will expect nothing less. My prayer for your success and safety will follow you wherever you happen to serve."