In this Letter, we proposed and experimentally demonstrated a directly modulated tunable laser based on the multi-wavelength distributed feedback (DFB) laser array. The lasers are placed in series to avoid the usage of an optical combiner and additional power loss. A three-section design is utilized to reduce the interference from other lasers and improve the electro-optic response bandwidth. Besides, the reconstruction-equivalent-chirp technique is used to simplify the grating fabrication and precisely control the grating phase. We realized 12 channels with 100 GHz spacing with high side mode suppression ratios of above 50 dB. The output power of all the channels is above 14 mW. The 3 dB electro-optic bandwidth is above 20 GHz at a bias current of 100 mA for all four lasers. A 25 Gb/s data transmission over a standard single-mode fiber of up to 10 km is demonstrated for all 12 channels, and 50 Gb/s data per wavelength is obtained through the four-level pulse amplitude modulation. The proposed directly modulated tunable in-series DFB laser array shows the potential for a compact and low-cost light source for wavelength division multiplexing (WDM) systems, such as next-generation front-haul networks and passive optical networks.