  CONTEXT. 2MASS is the reference survey in the NIR part of the spectrum given
its whole-sky coverage, large dynamic range, and proven calibration uniformity.
However, previous studies disagree in the value of the zero points (ZPs) for
its three bands JHK at the hundredth of a magnitude level. The disagreement
should become more noticeable now that Gaia provides whole-sky optical
photometry calibrated below that level. AIMS. We want to establish the value of
the 2MASS ZPs based on NICMOS/HST spectrophotometry of the CALSPEC standard
stars and test it with the help of Gaia DR2 parallaxes. METHODS. We have
computed the synthetic JHK photometry for a sample of stars using the HST
CALSPEC spectroscopic standards and compared it with their 2MASS magnitudes to
evaluate the ZPs. We have tested our results by analysing a sample of FGK
dwarfs with excellent 2MASS photometry and accurate Gaia DR2 parallaxes.}
RESULTS. The Vega ZPs for 2MASS J, H, and K are found to be -0.025$\pm$0.005
mag, 0.004$\pm$0.005 mag, and -0.015$\pm$0.005 mag, respectively. The analysis
of the FGK sample indicates that the new ZPs are more accurate than previous
ones.
