The spin and parity of the $Z_c(3900)^pm$ state are determined to be $J^P=1^+$ with a statistical significance larger than $7sigma$ over other quantum numbers in a partial wave analysis of the process $e^+e^-to pi^+pi^-J/psi$. We use a data sample of 1.92 fb$^{-1}$ accumulated at $sqrt{s}=4.23$ and 4.26 GeV with the BESIII experiment. When parameterizing the $Z_c(3900)^pm$ with a Flatte-like formula, we determine its pole mass $M_textrm{pole}=(3881.2pm4.2_textrm{stat}pm52.7_textrm{syst})textrm{MeV}/c^2$ and pole width $Gamma_textrm{pole}=(51.8pm4.6_textrm{stat}pm36.0_textrm{syst})textrm{MeV}$. We also measure cross sections for the process $e^+e^-to Z_c(3900)^+pi^-+c.c.to J/psipi^+pi^-$ and determine an upper limit at the 90% confidence level for the process $e^+e^-to Z_c(4020)^+pi^-+c.c.to J/psipi^+pi^-$.
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