As far as I know there is currently no evidence that it is a topological insulator - at least none of the 22 articles currently on arXiv containing phosphorene in the title mention anything of the sort.
But could it be? Well, I wouldn't get my hopes up at the moment. A key requirement for a topological insulator is band inversion (it is a necessary, but not sufficient condition), and first-principles calculations of the phosphorene band structure seem to suggest ordinary semiconductor gaps. See e.g. fig. 1 in Liu et al. (2014).
Now, sometimes a band inversion can be driven by spin-orbit coupling (likely a weak factor for Phosphorous, given the low atomic mass), doping (as in silicene) or strain (as in stanene). At this point, I wouldn't want to rule out the possibility that it will be found in phospherene too. That said, Peng et al. (2014) have already provided DFT calculations for phosphorene under strain, and don't seem to find any inversion.