The decision was ratified at a league AGM on Thursday night.
The Old Firm will be able to field second string sides in the fifth tier “for one year only” and they won’t be able to earn promotion to League Two.
A majority had previously indicated their support of the proposals.
The inclusion of the ‘B’ teams, it is hoped, will increase the exposure of the division, while netting each club a reported £3,000.
A statement from the league read: “The Scottish Lowland Football League can confirm that the majority of member clubs have voted in favour of admitting Rangers and Celtic ‘B’ teams into the league next season – for one year only.
“The decision was confirmed at our AGM tonight.
“Earlier this month all member clubs took part in a discussion with representatives from both Celtic and Rangers before an indicative vote.”
Celtic and Rangers will take the Lowland League up to 18 teams following the promotion of Kelty Hearts who defeated Brechin City in the Pyramid Play-off final.
The new campaign is expected to start as early as July 17.
George Fraser, the league's chair, said: “The member clubs have voted in favour of this proposal that was put to them earlier this month.
“Whilst it wasn’t unanimous the majority of member clubs are in favour of it and ultimately the members made the decision, which is only right in a democracy.
“Our Board will now get to work on moving forward with plans to get the new season started on July 17.”