Behavioural finance arose due to the failure of modern financial theories [capital asset pricing model (CAPM), efficient market hypothesis and arbitrage pricing theory (APT)] to offer answers to the anomalies that were being experienced in the financial markets. Behavioural finance is the incorporation of psychology and sociology in the financial decision making. Studies have been conducted to establish the behavioural factors that influence investment decisions but they are not conclusive. Investments decisions by teachers are critical because of the need to increase the income they earn and hence the need to make good investment decisions. Investment decisions are postulated to be affected by individual behavioural factors. Therefore, the paper was to determine the behavioural factors influencing high school teachers’ investment decisions in the stock market. The study employed phenomenology design. The population of the study was all the high school teachers investing in stocks in the Nairobi Securities Exchange in Nakuru County. Purposeful sampling was used to select the sample of the study. The study used 3 participants and one focus group. An unstructured in-depth interview schedule was used to elicit the behavioural factors that influence the investment decisions of high school teachers in the Nairobi securities exchange. Reduction process was used to analyse the data that was extracted from the in-depth interview of the participants. The study showed that high school teachers are affected by the behavioural factors while making investment decisions. The factors included herding behaviour, availability bias and loss aversion. The findings of this study will enable individual investors identify the behavioural factors affecting their investment decisions and hence be in a position to improve on their financial decision making which will ultimately enhance their performance in the stock market. Future researchers would also benefit from the study by using it as the basis for future related studies.