Abstract: This study investigates the factors that have influenced the share investment decisions of a sample of 2000 Nigerians. It is motivated by the observed significant investment in shares in Nigeria in recent years as well as the need to understand the behaviour of investors in Nigerian capital market and the factors that influence them to invest in shares. The theoretical anchor for is the behavioural finance theory. Based on this theory, a set of 20 factors cutting across social, economic, psychological, cultural issues that could influence investment decisions in shares as suggested by the available literature were identified and organized alongside with bio-data information to make a questionnaire which was then used to conduct the investigation. The data collected were analysed using both descriptive and inferential statistical analyses. The results obtained show that the top 10 factors that have influenced the investment decisions of the respondents in order of ranking are motivation by people who have attained financial security through share investment, future financial security, recommendations by reputable and trusted stock brokers, management team of the company, awareness of the prospects of investing in shares, composition of board of directors of companies, recent financial performance of the company, ownership structure of the company, reputable predictions of future increment in share value and bonus payments in order of mentioning. On the average, the group of factors making up social factors pooled the highest scores. This is followed by economic factors in the 2nd position. Psychological factors come 3rd while the 4th and the least in scores is cultural factors. However, the observed differences in the scores among the different sub-groups of factors were not significant when subjected to statistical analysis by means of analysis of variance. The study therefore concluded that investment decisions in shares are influenced jointly by economic, social, cultural and psychological factors.