A study was conducted in the net house of Department of Agronomy, Bangladesh Agricultural University as well as in farmers’ fields at Digarkanda village of Mymensingh district to evaluate the in situ and ex situ floristic diversity of the weed seed bank in rice. Five fields were surveyed for in situ evaluation with four replications and soil samples (1.5 kg soil) were collected and placed in plastic pots in the net house for ex situ study. Diversity was computed by the Shannon index (H’). a total of 33 weed species belonging to 17 families were found under in situ whereas, 37 species belonging to 22 families germinated under ex situ condition. The family Cyperaceae had the highest species richness and density under both conditions. Based on importance value, the five most dominant species under in situ condition were Eleocharis atroperpurea,Cyperus difformis, Alternanthera philoxeroides, Azolla pinnata and Echinochloa crusgalli. Whereas, under ex situ condition, two new weed species i.e. Fimbristylis miliaceae and Lindernia antipoda were found dominant instead of Alternanthera philoxeroides and Azolla pinnata and rest of the three species remained the same with slightly different rank and order. Weed density and diversity were also higher under ex situ condition than in situ condition. Ex situ condition had higher H’ index (H’=2.396) than in situ condition (H’=2.230). The highest percentage of weed emergence was observed within the first month of commencement of germination trial under both in situ and ex situ conditions. The information obtained from the study would help to determine the infestation potential of identified weed species and predict the upcoming threat which could lead to construct and improve successful weed management strategies.