We present deep observations of the field of the magnetar CXO J164710.2 - 455216 in the star cluster Westerlund 1 , obtained in the near-infrared with the adaptive optics camera NACO @ VLT . We detected a possible candidate counterpart at the Chandra position of the magnetar , of magnitudes \mathrm { J } = 23.5 \pm 0.2 , \mathrm { H } = 21.0 \pm 0.1 , and \mathrm { K } _ { \mathrm { S } } = 20.4 \pm 0.1 . The K _ { S } -band measurements available for two epochs ( 2006 and 2013 ) do not show significant signs of variability but only a marginal indication that the flux varied ( at the 2 \sigma level ) , consistent with the fact that the observations were taken when CXO J164710.2 - 455216 was in quiescence . At the same time , we also present colour–magnitude and colour–colour diagrams in the J , H , and K _ { S } bands from the 2006 epoch only , the only one with observations in all three bands , showing that the candidate counterpart lies in the main bulk of objects describing a relatively well–defined sequence . Therefore , based on its colours and lack of variability , we can not yet associate the candidate counterpart to CXO J164710.2 - 455216 . Future near-infrared observations of the field , following-up a source outburst , would be crucial to confirm the association from the detection of near-infrared variability and colour evolution .