<p>The nationwide COVID-19 pandemic lockdown helps to improve the overall health of the environment due to the sudden reduction in anthropogenic activities in various economic Sectors. The river Ganga is historically significant and a source of life for a very large population in northern and central India, but in the past few years, the water quality has declined as a result of untreated industrial effluents and municipal sewage. The objective of this study is to examine the water quality of river Ganga in Bihar during the COVID-19 pre-lockdown (i.e., January–February), lockdown (i.e., April), and post-lockdown (i.e., May–June) period and its mapping using IDW method in Arc-GIS software. A total of 135 samples were collected from 27 sampling points covering different districts and towns of Bihar. Physico-chemical water quality parameters like pH, dissolved oxygen (DO), chloride, nitrate, fluoride, sulfate, biochemical oxygen demand (BOD), total dissolved solids (TDS), and total coliform (TC) were analyzed for all samples. From the sample, analysis result obtained it has been revealed a huge reduction of total coliform was recorded during the lockdown period. As per the WQI assessment, the overall water quality was “poor” during the pre-lockdown and “good” during the lockdown and post-lockdown periods. WPI shows the water quality was highly polluted, good, and moderately polluted during pre-lockdown, lockdown, and post-lockdown periods respectively. CPI value, of the water quality, was assessed as moderately polluted during pre-lockdown and slightly polluted during both the lockdown and post-lockdown period. Overall the quality of Ganga water has noticeable improvement during the lockdown indicating that the problem of water quality deterioration is largely anthropogenic.</p>