Aquaporins (AQPs) are multifunctional membrane channels and facilitate the transport of water across plant cell membranes. Among the plant AQPs, plasma membrane intrinsic proteins (PIPs), which cluster in two phylogenetic groups (PIP1 and PIP2), play a key role in plant growth. Our previous work has indicated that RhPIP2;1, a member of PIP2, is involved in ethylene-regulated cell expansion of rose petals. However, whether PIP1s also play a role in petal expansion is still unclear. Here, we identified RhPIP1;1, a PIP1 subfamily member, from 18 PIPs assemble transcripts in rose microarray database responsive to ethylene. RhPIP1;1 was rapidly and significantly down-regulated by ethylene treatment. RhETRs-silencing also clearly decreased the expression of RhPIP1;1 in rose petals. The activity of the RhPIP1;1 promoter was repressed by ethylene in rosettes and roots of Arabidopsis. RhPIP1;1 is mainly localized on endoplasmic reticulum and plasma membrane. We demonstrated that RhPIP1;1-silencing significantly inhibited the expansion of petals with decreased petal size and cell area, as well as reduced fresh weight when compared to controls. Expression of RhPIP1;1 in Xenopus oocytes indicated that RhPIP1;1 was inactive in terms of water transport, while coexpression of RhPIP1;1 with the functional RhPIP2;1 led to a significant increase in plasma membrane permeability. Yeast growth, β-Galactosidase activity, bimolecular fluorescence complementation, and colocalization assay proved existence of the interaction between RhPIP1;1 and RhPIP2;1. We argue that RhPIP1;1 plays an important role in ethylene-regulated petal cell expansion, at least partially through the interaction with RhPIP2;1.