A measurement of the $B^{0}$ meson lifetime using $ B^{0} \rightarrow J/\psi K^{*0} $ decays in data from 13 $\text {TeV}$ proton–proton collisions with an integrated luminosity of $ 140~\mathrm {fb^{-1}} $ recorded by the ATLAS detector at the LHC is presented. The measured effective lifetime is $ \tau = 1.5053\pm 0.0012~\mathrm {(stat.)} \pm 0.0035~\mathrm {(syst.)~ps}. $The average decay width extracted from the effective lifetime, using parameters from external sources, is $\begin{aligned} \Gamma _d = 0.6639\pm 0.0005~\mathrm {(stat.)} \pm 0.0016~\mathrm {(syst.)}\\ \pm 0.0038~\text {(ext.)} \text {~ps}^{-1}, \end{aligned}$where the uncertainties are statistical, systematic and from external sources. The earlier ATLAS measurement of $\Gamma _s$ in the $B^{0}_{s} \rightarrow J/\psi \phi $ decay was used to derive a value for the ratio of the average decay widths $\Gamma _d$ and $\Gamma _s$ for $B^{0} $ and $B^{0}_{s} $ mesons respectively, of $ \frac{\Gamma _d }{\Gamma _s } = 0.9905\pm 0.0022~\text {(stat.)} \pm 0.0036~\text {(syst.)} \pm 0.0057~\text {(ext.)}. $The measured lifetime, average decay width and decay width ratio are in agreement with theoretical predictions and with measurements by other experiments. This measurement provides the most precise result of the effective lifetime of the $B^{0}$ meson to date.