Carbon regulation is intended to reduce global emissions, but there is growing concern that such regulation may simply shift production to unregulated regions, potentially increasing overall carbon emissions in the process. Carbon tariffs have emerged as a possible mechanism to address this concern by imposing carbon costs on imports at the regulated region's border. Advocates claim that such a mechanism would level the playing field whereas opponents argue that such a tariff is anti-competitive. This paper analyzes how carbon tariffs affect technology choice, regional competitiveness, and global emissions through a model of imperfect competition between "domestic" (i.e., carbon-regulated) firms and "foreign" (i.e., unregulated) firms, where domestic firms have the option to offshore production and the number of foreign entrants is endogenous. Under a carbon tariff, results indicate that foreign firms would adopt clean technology at a lower emissions price than domestic producers, with the number of foreign entrants increasing in emissions price only over intervals where offshore foreign firms hold this technology advantage. Further, domestic firms would only offshore production under a carbon tariff to adopt technology strictly cleaner than technology utilized domestically. As a consequence, under a carbon tariff, foreign market share is non-monotonic in emissions price, and global emissions conditionally decrease. Without a carbon tariff, foreign share monotonically increases in emissions price, and a shift to offshore production results in a strict increase in global emissions.