This study examined the influence of psychological capital on market share of manufacturing firms in Nigeria. A cross-sectional survey research design was adopted for the study. Data were collected from 89 respondents using a structured questionnaire based on a five-point Likert scale. Descriptive statistics were used to analyse the responses, while simple linear regression was employed to test the hypotheses at the 0.05 level of significance. The findings revealed a general high level of optimism among employees, with positive responses indicating that employees maintain positive expectations, remain confident about organisational objectives and sustain positive outlooks during setbacks. The regression result showed that optimism had a strong, positive and statistically significant influence on market share. Based on these findings, the study concludes that psychological capital, particularly optimism and resilience, constitutes an important human resource capable of enhancing the market position of manufacturing firms in Nigeria. The study recommends that manufacturing firms should incorporate psychological-capital development into their human-resource management practices through training, mentoring, employee support, recognition and resilience-building programmes. Such interventions can strengthen employees’ ability to cope with challenges, sustain performance and contribute to improved market competitiveness.