The effects of bagging-induced light reductions on lycopene synthesis and the expression of related genes, antioxidant activity, and sugar composition of ‘Tubtim Siam’ pummelo (Citrus maxima) were investigated. Glucose, ascorbic acid, and flavonoid concentrations and 2, 2-diphenyl-1-picryhydrazyl scavenging activity were decreased in fruit covered with bags while still on the tree [0.01 μmol·m−2·s−1 photosynthetic photon flux density (PPFD)] compared with the untreated control (596.7 μmol·m−2·s−1 PPFD). The bagging treatment significantly decreased the temperatures on the surface in the bag. In addition, the bagging treatment decreased abscisic acid concentrations in the peel and pulp. However, the bagging treatment increased lycopene concentrations, upregulated phytoene synthase (CsPSY) and ζ-carotene desaturase (CsZDS) gene expressions; downregulated chromoplast-specific lycopene cyclase (CsβLCY), β-carotene hydroxylase (CsβCHX), and ε-ring hydroxylase (CsɛCHX); and decreased 9-cis-epoxycarotenoid dioxygenase (CsNCED1) gene expressions in the pulp. It is possible that maintaining a temperature of ≈25 °C in fruit covered with bags may increase the lycopene concentration in the pulp with the upregulation of CsPSY and CsZDS and the downregulation of CsβLCY, CsβCHX, CsɛCHX, and CsNCED1 gene expressions in the pulp.