From 1310.6×106 J/ψ and 448.1×106 ψ(3686) events collected with the BESIII experiment, we report the first observation of ς+ and ς¯-spin polarization in e+e-→J/ψ[ψ(3686)]→ς+ς¯-decays. The relative phases of the form factors Δφ have been measured to be (-15.5±0.7±0.5)° and (21.7±4.0±0.8)° with J/ψ and ψ(3686) data, respectively. The nonzero value of Δφ allows for a direct and simultaneous measurement of the decay asymmetry parameters of ς+→pπ0(α0=-0.998±0.037±0.009) and ς¯-→p¯π0(α¯0=0.990±0.037±0.011), the latter value being determined for the first time. The average decay asymmetry, (α0-α¯0)/2, is calculated to be-0.994±0.004±0.002. The CP asymmetry ACP,ς=(α0+α¯0)/(α0-α¯0)=-0.004±0.037±0.010 is extracted for the first time, and is found to be consistent with CP conservation.