The improved strength and ductility of fiber-reinforced polymer (FRP) confined concrete is a result of external confinement, which places an effective constraint on its lateral dilation. For a reliable confined concrete design, an understanding of the confinement mechanism and dilation of the concrete core, which can be achieved by studying its dilation characteristics, is crucial. In the absence of any experimental evidence, concrete behavior under the confinement of new materials remains unclear, and the direct application of existing confinement models developed for other FRPs may not be suitable. This paper discusses in detail the dilation characteristics of concrete confined by a polyethylene terephthalate (PET) FRP composite, which is a newly developed FRP with a bilinear stress-strain response, low elastic modulus, and large rupture strain (LRS). The effects of column parameters, such as the number of PET FRP layers, corner radius, and cross-section shape, are also evaluated. The experimental results show that under the soft confinement of PET FRP, the concrete core exhibited a significantly higher lateral dilation, Poisson’s ratio, dilation rate, and volumetric expansion compared to concrete confined by other FRPs. However, because of the LRS capacity, PET FRP controlled the unstable response efficiently and exhibited a remarkably long stable response, particularly in noncircular cross-sections. The results also indicate that the soft confinement of PET FRP could not curtail volumetric expansion but restrained it significantly under the effect of the considered parameters.