A measurement is reported of the jet mass distribution in hadronic decays of boosted top quarks produced in
p
p
collisions at
√
s
=
13
 
 
TeV
. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of
35.9
 
 
fb
−
1
. The measurement is performed in the
lepton
+
jets
channel of
t
¯
t
events, where the lepton is an electron or muon. The products of the hadronic top quark decay
t
→
b
W
→
b
q
¯
q
′
are reconstructed as a single jet with transverse momentum larger than 400 GeV. The
t
¯
t
cross section as a function of the jet mass is unfolded at the particle level and used to extract a value of the top quark mass of
172.6
±
2.5
 
 
GeV
. A novel jet reconstruction technique is used for the first time at the LHC, which improves the precision by a factor of 3 relative to an earlier measurement. This highlights the potential of measurements using boosted top quarks, where the new technique will enable future precision measurements.