  We present optical/near-infrared spectroscopy and photometry of the supernova
(SN) PS15si. This object was originally identified as a Type IIn SN, but here
we argue that it should be reclassified as a Type Ia SN with narrow hydrogen
lines originating from interaction with circumstellar matter (CSM; i.e., SN
Ia/IIn or SN Ia-CSM). Based on deep nondetections 27 days before discovery, we
infer that this SN was discovered around or slightly before optical maximum,
and we estimate the approximate time that it reached R-band maximum based on
comparison with other SNe Ia/IIn. In terms of spectral morphology, we find that
PS15si can be matched to a range of SN Ia spectral types, although SN
1991T-like SNe Ia provides the most self-consistent match. While this
observation agrees with analysis of most other SNe Ia/IIn, we find that the
implied CSM luminosity is too low to account for the overall luminosity of the
SN at a time when the CSM should outshine the underlying SN by a few
magnitudes. We infer that the similarity between PS15si and the hot,
overluminous, high-ionisation spectrum of SN 1991T is a consequence of a
spectrum that originates in ejecta layers that are heated by ultraviolet/X-ray
radiation from CSM interaction. In addition, PS15si may have rebrightened over
a short timescale in the B and V bands around 85 days after discovery, perhaps
indicating that the SN ejecta are interacting with a local enhancement in CSM
produced by clumps or a shell at large radii.
