Wilshere was investigated by European football's governing body following a series of tweets during a Champions League fixture against Olympiacos in December.
The midfielder, sidelined by injury, wrote on Twitter that team-mate Emmanuel Frimpong was “worth a cheeky £10” to score the first goal, before adding, when Frimpong had a shot: “Frimmy nearly won me some money there!”
Players are not permitted to place bets on games involving their own clubs. Wilshere later tweeted: “I didn’t actually bet on the game. I know we’re not allowed to! I was only messing just to be clear.”
However, UEFA has now decided to issue a formal warning to the Arsenal star.
“The player has been warned for being in breach of the principles of integrity of matches and competitions," a UEFA spokesman told The Sun, who alerted the organisation to Wilshere's tweets.
“The disciplinary measure was a warning.”