Gold nanostars (GNSs) have received considerable attention in surface-enhanced spectroscopies, catalysis, biosensing, photothermal therapy, and photovoltaics because of their unique optical properties arising from the anisotropic structure. GNSs typically consisting of a central core and several protruding tips are usually synthesized by a seed-mediated growth approach, but the growth mechanism and optical properties have yet to be fully understood. Here, we systematically investigate the seed-mediated growth process of GNSs to gain an insight into the growth mechanism and evolution of their optical and photothermal properties. By tailoring the core size, tip length and tip angle, the main localized surface plasmon resonance (LSPR) peak wavelength can be broadly tuned from the visible to near-infrared (NIR) region. Our observations show that the protruding tips grow rapidly away from the central core at the initial growth stage, leading to a red shift of the main LSPR peak. The preferential deposition of gold atoms onto the gold core takes place at the later growth stage, gradually blue-shifting the main LSPR peak. GNSs exhibit a large molar extinction coefficient ranging from 4.0 X 10(8) M-1 cm(-1) to 4.5 x 10(10) M-1 cm(-1), the log value of which correlates linearly with the main LSPR peak wavelength and accordingly allows for facile determination of the GNS concentration in a suspension. In addition, GNSs are excellent NIR photothermal materials with the LSPR-dependent photothermal conversion efficiency. The maximum photothermal conversion efficiency of GNSs occurs at a LSPR wavelength of 740 nm, blue-shifted from the incident laser wavelength. Our present work suggests that GNSs exhibit excellent optical and photothermal properties that can be optimized by tailoring the dimensional parameters.