Can you build muscle on a plant-only diet?

Short answer

Yes, if protein is covered. In trials where a vegan diet supplied about 112-140 g of protein a day for a 70 kg person, muscle gain did not differ from a meat diet. When 72 people aged 65 and over chose their own vegan food, protein fell by 19-28 g a day and muscle shrank.

Can you build muscle on a plant-only diet?
The formula: 'no meat → too little protein → no muscle'. Is it true?
A common claim is that without meat you run short of protein and muscle will not grow. We checked this formula with trials that changed the whole diet to plants, not just a single protein powder.
With protein matched, there was no difference
19 habitual vegans were compared with 19 meat eaters
19 habitual vegans and 19 meat eaters trained 2 times a week for 12 weeks. Missing protein was topped up with supplements so both ate about 112 g a day for a 70 kg (154 lb) person. Leg lean mass (muscle and everything except fat) rose 1.2 kg in both groups, with no difference.
Switching the whole diet to vegan gave the same result
Thigh muscle volume rose 8.3% in both groups
22 people trained 5 times a week for 10 weeks on either a diet with meat or a diet with no animal foods. Both ate about 140 g of protein a day for a 70 kg person. Thigh muscle volume rose 8.3% in both groups. The vegan group got most of its protein from a fungus-based protein (mycoprotein).
But self-chosen plant diets had less protein
In a trial of people aged 65 and over, intake fell by 19-28 g a day
A review of 141 studies found that people on plant-based diets ate less protein than meat eaters, though still within recommended amounts. In a trial that randomly split 72 people aged 65 and over, letting people choose their own vegan food cut protein by 19-28 g a day.
The vegan group lost muscle too
Even with strength training it shrank
In the same trial of 72 people aged 65 and over, thigh muscle volume after 12 weeks fell 0.61 L on the vegan diet and 0.01 L on the usual diet. The vegan group that also did strength training still lost 0.35 L. Arm and leg lean mass fell 1.15 kg on the vegan diet and rose 0.05 kg on the usual diet.
The same plant diet changed when protein was set
Self-chosen plant diets lost muscle; set soy protein showed no difference
Among 19 people in their 50s and 60s who trained for 12 weeks, those who chose their own vegetarian food lost lean mass while meat eaters gained. In a trial of 21 people averaging 65, soy protein was assigned and total protein was about 72-82 g a day for a 70 kg person. Thigh muscle rose 4.2% with soy and 6.0% with beef, with no difference.
Summary: plant diets build muscle, if protein is covered
Dropping meat cuts protein first
Vegan diets with about 112-140 g of protein a day for a 70 kg person showed no difference in muscle gain from meat diets. When 72 people aged 65 and over chose their own vegan food, protein fell by 19-28 g a day, and muscle shrank even with training. On a plant diet, counting protein first is the way to go.