Behavioural biases describe a replicable pattern in perceptual distortion, inaccurate judgment, illogical interpretation, or what is broadly called irrationality. This paper adopts a primary data approach to investigate the effects of behavioural biases on security market performance in Nigeria. The objectives are in twofold: one, to examine the extent of behavioural biases among security market investors in Nigeria and, to examine the effects of behavioural biases on stock market performance in Nigeria. The paper employed questionnaire as instrument and the technique of correlation with Pearson Product Moment Coefficient to analyze a survey of 300 randomly selected investors in Nigeria security market. We find strong evidence that behavioural biases exists but not so dominant in the Nigeria security market because a weak negative relationship exists between behavioural biases and stock market performance in Nigeria. The paper recommends that individual investors in the market should engage the services of investment advisors which will reduce personal biases in the management of their portfolios.