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We present photometric and spectroscopic data of the unusual interactingsupernova (SN) 2021foa. It rose to an absolute magnitude peak of $M_r=-18$ magin 20 days. The initial light curve decline shows some luminosity fluctuationsbefore a long-lasting flattening. A faint source ($M_r\sim -14$ mag) wasdetected in the weeks preceding the main event, showing a slow-risingluminosity trend. The $r$-band absolute light curve is very similar to those ofSN 2009ip-like events, with a faint and shorter duration brightening (`EventA') followed by a much brighter peak (`Event B'). The early spectra of SN2021foa show a blue continuum with narrow ($v_{FWHM}\sim$400 km s$^{-1}$) Hemission lines, that, two weeks later, reveal a complex profile, with a narrowP Cygni on top of an intermediate-width ($v_{FWHM}\sim$2700 km s$^{-1}$)component. At +12 days metal lines in emission appear, while \Hei lines becomevery strong, with \Hei~$\lambda$5876 reaching half of the \Ha luminosity, muchhigher than in previous SN 2009ip-like objects. We propose SN 2021foa to be atransitional event between the H-rich SN 2009ip-like SNe and the He-rich TypeIbn SNe.

SN 2021foa, a transitional event between a Type IIn (SN 2009ip-like) and a Type Ibn supernova