Justin Bieber -- SEARCHED In Puerto Rico

Justin Bieber was searched at the airport in San Juan, Puerto Rico as he prepared to leave and security agents rifled through his bags, looking for contraband ... TMZ has learned.

Law enforcement sources tell TMZ ... security at the airport claimed it was a "random search." It doesn't seem random, because we're also told they lasered in on Biebs because of the large amount of luggage.