  The COMPASS experiment at CERN, as part of its programme addresses the
exploration of the transverse spin structure of the nucleon by measuring spin
(in)dependent azimuthal asymmetries in semi-inclusive DIS and, recently, also
in Drell-Yan processes. Between 2002 and 2010 COMPASS performed a series of
SIDIS measurements, using a longitudinally polarized muon beam impinging on
transversely polarized $^6LiD$ or $NH_3$ targets. Drell-Yan measurements with a
$\pi^-$ beam interacting with a transversely polarized $NH_3$ target started
with the 2015 run and will be continued in 2018. In this Letter the first
measurement of transverse-spin-dependent azimuthal asymmetries in the
pion-induced Drell-Yan process is reported. Measured asymmetries giving access
to different transverse-momentum-dependent (TMD) parton distribution functions
(PDFs) are extracted using dimuon events with invariant mass between
4.3~\gvcw\, and 8.5~\gvcw. A recent COMPASS SIDIS measurement was obtained at a
hard scale comparable to that of these DY results. This provides a unique
possibility to test predicted in QCD sign change of the Sivers TMD PDF and
other (pseudo-)universal features of transvers momentum dependent parton
distribution functions.
