We report on the electron spin resonance (ESR) properties of a van der Waals bulk CrBr3 single crystal. Angular- and temperature-dependent studies are performed in the range of the critical temperature for ferromagnetic order. The angular-dependent data enable the determination of a strong uniaxial magnetic anisotropy constant, K, with the easy axis of magnetization being parallel to the crystallographic c axis. The peak-to-peak ESR linewidth below the Curie temperature shows contributions from the term (3 cos2 θ−1)2, thus suggesting the occurrence of long-wavelength (q → 0) modes of spin fluctuations typical for 2D systems. Finally, we analyze the temperature dependence of the double integrated ESR intensity, resonance field, and linewidth for the direction of the magnetic field parallel and perpendicular to the c easy axis. We determine Weiss's constant and the thermal dependence of K and observe the …