The fibularis tertius is a small muscle in the lower leg that helps control your foot and ankle. It sits on the front outer side of your lower leg and its tendon runs down to attach on the top of the foot, near the base of the little-toe side. When it contracts, it helps lift the front of your foot upward at the ankle and it also helps turn the sole of your foot slightly outward. Those actions matter most when you walk, run, change direction, or land from a jump, because they help position the foot so it clears the ground during the swing phase and stays stable when it contacts the floor.
Not everyone has a fibularis tertius, and that can be completely normal. In some people it is well developed, in others it is very small, and in others it is absent. When it is present, it works closely with other muscles on the front of the shin that lift the foot, and with the muscles on the outer lower leg that help guide the ankle. Think of it as a helper muscle that fine tunes ankle control, especially on uneven surfaces and during quick movements.
For training, it is rarely something you isolate directly, but it gets challenged during activities like brisk walking, running, jump rope, agility drills, and strength work that includes controlled ankle movement. If the tendon gets irritated, you may feel discomfort on the front outer ankle or top outer foot, especially with repeated running or sudden increases in volume. In that case, backing off impact, improving ankle mobility and calf strength, and progressing training gradually usually helps.