Pyropia aquaculture industries have grown in a recent year, but information is still lacking regarding cultivation techniques for early life stages. In this study, photosynthetic performance and growth of Pyropia dentata in the conchocelis phase was investigated under various culture conditions. Specifically, we used culture conditions with different temperature (10, 15, and 20 °C), light intensity (20 and 70 μmol photons m−2 s−1), and light type (LEDBlue, LEDRed, LEDBlue+Red, and fluorescent lamp) combinations. Photosynthesis was measured using chlorophyll fluorometry (Mini-PAM) after 3 weeks of acclimation under various experimental conditions. Additionally, image analysis was used to determine growth and estimate the 2-dimensional area of complicated conchocelis structures. Results showed that growth rates significantly increased under higher temperatures (15 and 20 °C), but light intensity did not significantly influence photosynthesis and growth. LEDBlue+Red and LEDBlue treatments had higher growth rates and photosynthetic efficiency than LEDRed and fluorescent lamp treatments after the 3-week acclimation period. However, higher photochemical capacity (photosynthesis) was clearly observed in the low temperature condition; thus, depressed growth rates in the low temperature condition are not indicative of physiological senescence of P. dentata conchocelis. Based on these results, we established optimized laboratory cultivation conditions for the conchocelis phase of P. dentata. Specifically, we found that this species requires a higher cultivation temperature to ensure optimal growth. Also, mixing red and blue or solitary blue light sources could be an efficient stabilized laboratory cultivation technique.