Sr2NaOsO6, an ordered double perovskite, was obtained by solid‐state synthesis. The compound crystallizes in a monoclinic cell P21/n, a = 5.7043(1) Å, b = 5.7828(1) Å, c = 8.0816(2) Å; β = 90.01(2)° and develops long‐range antiferromagnetic order at ≈ 17 K with strong canting generating a tiny spontaneous moment of 0.06 µB/Os ion. A fit of the high temperature susceptibility data according to a modified Curie–Weiss law revealed a small effective paramagnetic moment of 0.7 µB/Os, which underlines the substantial effect of spin‐orbit coupling in 5d oxides. A broad instead of a sharp lambda anomaly in specific heat data suggests frustration and strong magnetic disorder at the osmium lattice. A new B‐site ordered, monoclinic double perovskite Sr2NaOsO6 is reported. It exhibits a long range antiferromagnet order below TN = 18 K but with strong spin canting. Magnetic and calorimetric studies suggest a considerable disorder at the magnetic lattice.