In a data sample of approximately 1.3 fb−1 collected with the D0 detector between 2002 and 2006, the orbitally excited charm state Ds1±(2536) has been observed with a measured mass of 2535.7±0.6(stat)±0.5(syst) MeV/c2 via the decay mode Bs0→Ds1−(2536)μ+νμX. A first measurement is made of the branching ratio product Br(b¯→Ds1−(2536)μ+νμX)×Br(Ds1−→D*−KS0). Assuming that Ds1−(2536) production in semileptonic decay is entirely from Bs0, an extraction of the semileptonic branching ratio Br(Bs0→Ds1−(2536)μ+νμX) is made.Received 26 December 2007DOI:https://doi.org/10.1103/PhysRevLett.102.051801©2009 American Physical Society