Can bicycling help me gain muscle mass?

Yes: In the legs, make sure to hit the big hills!

Degree: To a degree. It may simply tone what you have. Much depends on how you are cycling--easy cruiser Sunday riding or road bike in the drops full on pace. Arm position also a factor. Clearly if you are merely holding on to upright wide bars to balance and steer it is different than if you are in the drops supporting the body and flexing for leverage and power. Both are enjoyable, but different results

Of course: Cycling will help build your calves, thighs, and hamstrings. Good luck!

Effects of Exercise: Gaining muscle mass is about muscle hypertrophy or muscle size. This is done by strength training or using your muscles against a heavy load. On the other hand bicycling is more of an aerobic activity and increases the number of blood vessels in a muscle without increasing its size significantly. The increased flow is more efficient for the delivery of glucose and oxygen.