  In this paper, we study lensing of 21cm intensity mapping (IM). Like in the
cosmic microwave background (CMB), there is no first order lensing in intensity
mapping. The first effects in the power spectrum are therefore of second and
third order. Despite this, lensing of the CMB power spectrum is an important
effect that needs to be taken into account, which motivates the study of the
impact of lensing on the IM power spectrum. We derive a general formula up to
third order in perturbation theory including all the terms with two derivatives
of the gravitational potential, i.e. the dominant terms on sub-Hubble scales.
We then show that in intensity mapping there is a new lensing term which is not
present in the CMB. We obtain that the signal-to-noise of 21 cm lensing for
futuristic surveys like SKA2 is about 10. We find that surveys probing only
large scales, lmax < 700, can safely neglect the lensing of the intensity
mapping power spectrum, but that otherwise this effect should be included.
