Behavioural finance integrates psychology and finance to explain how emotions, cognitive biases, and behavioural tendencies influence investment decisions. This study examines the impact of behavioural finance and investor psychology on investment decisions with reference to Tata Motors. It analyses how biases such as overconfidence, herd behaviour, loss aversion, anchoring, and confirmation bias affect investors' buying, holding, and selling decisions. Primary data were collected through a structured questionnaire from individual investors, while secondary information was obtained from books, journals, company reports, and financial publications. The findings indicate that behavioural biases significantly influence investment decisions, particularly during periods of market volatility. The study concludes that improving financial literacy, emotional discipline, and awareness of behavioural biases enables investors to make more rational investment decisions and achieve better long-term portfolio performance.