Recent research suggests the emergence of simultaneous Puerto Rican Spanish-English bilinguals in younger generations of speakers (Fabiano-Smith et al., 2014). With the emergence of larger numbers of L1 English speakers on the island, it is reasonable to expect subsequent dialect formation, not as part of a generational language shift, but rather as a reflection of complex cultural interactions and the amelioration of language attitudes towards American English. This dissertation employs approaches and concepts including perceptual dialectology, signal detection theory, and rootedness to explore the process of enregisterment and new dialect formation in Puerto Rican Island English (PRIE) as an emerging variety of American English. A total of 338 naïve listeners, mainly from the Upper Midwest, responded to three surveys: (1) a Matched-Guise Box Task, (2) a set of Identification and AX Judgment Tasks, and (3) a Mental Map Task. Nonparametric statistical analyses were conducted for the Judgment Tasks and comparative descriptive analyses were conducted on the results in the Mental Map Task. The findings in the signal detection tasks confirmed that listeners identified PRIE as distinct from the other speaker groups, and that PRIE holds a similar perceptual status as the other simultaneous Spanish-English bilingual varieties of mainland American English. The results from the Mental Map Task indicated that PRIE did not yet appear as a form that was uniquely enregistered to Puerto Rico. Perceptions of PRIE speakers patterned closer with that of the mainland simultaneous bilingual speaker groups that were most associated to the English controls. Puerto Rico has been almost entirely viewed as an L1 Spanish/L2 English-speaking community, even with the increasing sociocultural and sociopolitical influence of the mainland United States on the island. This dissertation is the first study to show the rise of a new variety of American English in Puerto Rico that can be heard by listeners in the mainland United States. The findings in this study begin to reshape the conversation on the role of English on the island and dispel some of the myths associated with a lack of knowledge about the linguistic diversity and changing linguistic landscape of Puerto Rico.