With the rapid development of the global nuclear energy industry and the implementation of plans to discharge treated radioactive water from Fukushima, Japan, into the sea, the transport of tritium (3H) in coastal waters and its environmental impacts have attracted significant attention. In this study, we collected a series of rainwater, river water, groundwater, interstitial water, and seawater samples to determine 3H in the Changjiang Estuary area in China in 2023 and 2024. The distribution of 3H activity in the rivers in Shanghai was uniform, ranging from 5.41 to 8.83 TU (1 TU = 0.118 Bq L−1). The activity of 3H in rainwater samples in Shanghai showed a similar range, varying from 3.15 to 9.82 TU. The activities of 3H in groundwater, interstitial water, and seawater in Northern Chongming Tidal Flat were lower, with values of 3.99–5.41, 1.61–5.16, and 0.76–3.26 TU, respectively. Off the Changjiang Estuary, the 3H activity in seawater gradually decreased with increasing offshore distance and was dominated by the terrestrial 3H input carried by Changjiang runoff. The 3H flux from the Changjiang River to the estuary was 19.7× 1012 Bq d−1. The flux of 3H transported by submarine groundwater discharge in the Changjiang Estuary was 3.11× 1011 Bq d−1, accounting for approximately 25.8 % of the river transport. These findings indicate that seabed groundwater is an indispensable contributor to 3H balance in the Changjiang Estuary. The groundwater renewal rate on Chongming Island is relatively fast, with an age of approximately 3.5 years.