According to a White House release, they will be accompanied by unspecified "long-time family friends." President Obama will not join, and neither will their older daughter Malia, according to a report.
The first lady will make an official visit to King Juan Carlos and Queen Sophia of Spain per their invitation, but all activities on the trip are closed to the press.
The trip coincides with President Obama's birthday, which is Aug. 4. The day after, Obama will be in Michigan and his hometown of Chicago visiting auto plants.
Last June, the first lady privately vacationed with both of her daughters and mother in France and England. They visited French President Nicolas Sarkozy and his wife Carla and later met with Sarah Brown, the wife of then-British Prime Minister Gordon Brown.
Michelle ObamaMichelle LeVaughn Robinson ObamaMichelle Obama looks to mobilize voters for midterms We must mount an all-country response to help our Afghan allies Obamas, Bushes and Clintons joining new effort to help Afghan refugees MORE also took Sasha and Malia to the "Harry Potter" movie set for the former's birthday.