  We calculate the massless quark jet function to three-loop order. The quark
jet function is a universal ingredient in SCET factorization for many collider
and decay processes with quark initiated final state jets. Our three-loop
result contributes to the resummation for observables probing the invariant
mass of final state quark jets at N$^3$LL$^\prime$. It represents the first
complete three-loop result for a factorization ingredient describing collinear
radiation. Furthermore it constitutes a major component of the $N$-jettiness
subtraction/slicing method at N$^3$LO, which eventually may enable the
calculation of fully-differential cross sections with a colorful final state at
this order.
