Manufacturing firms\ productivity has always been the panacea for sustainable economic growth and development. Nigerian economic growth has been very slow due to the abysmally low level of manufacturing sector productivity. In an attempt to promote the sub-sector for high level of productivity there have been several developmental plans and policies implemented by successive governments in Nigeria to deepen the financial system. Despite these efforts, it seems the manufacturing firms have not achieved the desired level of productivity. Based on the foregoing, this study investigated the effect of financial deepening on manufacturing firms’ productivity in Nigeria. The data from 1986 to 2017, used for the study, were sourced from the publications of the Central Bank of Nigeria and National Bureau of Statistics. Autoregressive Distributed Lag model was employed to produce the parameter estimates. The validity of the results was examined using Breusch-Godfrey Serial Correlation and Breusch-Pagan-Godfrey heteroscedasticity tests. The findings reveal that financial deepening cointegrate, and has a significant negative effect on the index of manufacturing production in Nigeria. The major implication of this finding is that financial deepening has significant negative effect on manufacturing sector productivity in Nigeria. It is therefore recommended among others that government shouldrework and re-align the various financial deepening strategies to be properly adapted to the Nigerian financial peculiarities by promotingfinancial deepening strategies that relax capital market entry requirements to encourage small and medium scale manufacturing firms to be listed on the Nigerian Stock Exchange.