The study aimed at evaluating the effects of high ambient temperature (HT: 30 °C) on the thermoregulatory responses and performance of commercial and Piau crossbred (Brazilian Piau breed sires × commercial genotype dams) growing pigs. Commercial and Piau crossbred pigs were reared under thermoneutral (TN: 22 °C) or HT conditions during a 14-day experimental period. Feeding (daily) and animals (beginning and end) were weighted to obtain performance parameters. Skin and rectal temperatures, respiratory rate, and blood parameters were also measured. At the end of the trial (day 15), the animal’s backfat thickness (BF) and loin eye area (LEA) were measured. No interaction (p > 0.05) between the genetic group and ambient temperature was observed for any performance trait. Irrespective of ambient temperature, Piau crossbred pigs had a similar feed intake (ADFI, 2615 g/day, on average; p > 0.05), lower daily weight gain (ADG, −234 g/day; p < 0.01), and a higher feed conversion ratio (FCR, +0.675 g/g; p < 0.01). There was interaction (p = 0.01) between genotype and ambient temperature for the LEA that decreased significantly in response to HT in commercial pigs (−6.88 cm2) and did not differ in response to ambient temperature in Piau crossbred pigs (29.14 cm2, on average; p > 0.05). Piau crossbred pigs had greater BF (+7.2 mm; p < 0.01) than commercial pigs. Regardless of the genetic group, exposure of pigs to HT resulted in decreased ADFI (−372 g/day; p < 0.01), ADG (−185 g/day; p < 0.01), and a higher FCR (+0.48 g/g; p = 0.01). Ambient temperature did not affect lipid deposition. Pigs at HT had an increased respiratory rate (+38 bpm; p < 0.01) and a long-lasting increase in skin and rectal temperatures compared to TN pigs. Total concentrations of triiodothyronine (T3) and thyroxine (T4) were not affected by ambient temperature in commercial pigs, whereas Piau crossbred pigs kept at 30 °C had a transient decrease in both hormones at day 2 (p < 0.01). Serum cortisol concentrations were not affected (p > 0.05) by genotype nor ambient temperature. In summary, Piau crossbred pigs had lower efficiency using nutrients for growth in association with increased lipid deposition when compared to commercial pigs. In response to HT, commercial pigs had a decreased LEA, whereas no effect was observed for Piau crossbred pigs. Apart from that, commercial and Piau crossbred pigs had a similar magnitude of thermoregulatory responses activation in response to HT, evidencing their innate survival-oriented function.