Bale has been restricted to nine appearances in all competitions for Madrid this season and last featured at Borussia Dortmund in the UEFA Champions League at the end of September – limping out of the action during the closing stages after opening the scoring in a 3-1 win.
On that occasion, the 28-year-old suffered a calf injury that ruled him out of Wales's decisive 2018 FIFA World Cup qualification defeat to the Republic of Ireland last month.
Bale, who had his 2016-2017 campaign disrupted by calf and ankle problems, was back in training with Zinedine Zidane's squad ahead of next weekend's derby at Atletico Madrid but a club statement confirmed he has strained an adductor muscle in his left leg.
A statement released by Madrid read: "Our player Gareth Bale, who felt some discomfort in his left leg at the end of Thursday's training session, today underwent various tests carried out by the Real Madrid Sanitas medical team.
"He has been diagnosed with a strain to the middle third of the adductor longus muscle in his left leg. His recovery will continue to be assessed."
Madrid did not put a date on Bale's return, simply stating his progress would be monitored, but reports in Spain suggest the former Tottenham Hotspur star is facing between three and four weeks on the sidelines.
Along with the trip to Atleti's Wanda Metropolitano on November 18, Bale will sit out Madrid's next Champions League match against APOEL and may also miss the conclusion of their Group H campaign when Dortmund visit the Santiago Bernabeu on December 6.
A trip to Athletic Bilbao and a home match against Sevilla in LaLiga come either side of the latter contest before Madrid begin their FIFA Club World Cup campaign in Abu Dhabi on December 13.
Zidane's men host Barcelona, who are eight points better off at the summit of Spain's top flight, in the first El Clasico of the season on December 23.