Weyl semimetals (WSMs) evolve from Dirac semimetals in the presence of broken time-reversal symmetry (TRS) or space-inversion symmetry. The WSM phases in TaAs-class materials and photonic crystals are due to the loss of spaceinversion symmetry. For TRS-breakingWSMs, despite numerous theoretical and experimental efforts, fewexamples have been reported. In this Article, we report a newtype of magnetic semimetal Sr1-yMn1-zSb2 (y, z < 0.1) with nearly massless relativistic fermion behaviour (m* = D 0.04-0.05m(0), where m0 is the free-electron mass). This material exhibits a ferromagnetic order for 304 K < T < 565 K, but a canted antiferromagnetic order with a ferromagnetic component for T < 304 K. The combination of relativistic fermion behaviour and ferromagnetism in Sr(1-y)Mn(1-z)S(b)2 offers a rare opportunity to investigate the interplay between relativistic fermions and spontaneous TRS breaking.