This study aims to investigate the interaction effects of marketing and R&D expenditure on brand competitiveness based on performance indicators. While many studies have investigated the individual effects of marketing expenditure and R&D expenditure on a company's brand value, competitive advantage, and performance, there has been limited research on the interaction effect of these two indicators on brand competitiveness. Longitudinal data were collected from 145 companies over seven years, including 1015 observations. The companies were selected using the systematic removal method from 485 companies on the Tehran stock exchange market. We used a generalised method of moments (GMM) to analyse the data. Findings demonstrate that marketing, when considered independently, had a significant effect on brand competitiveness (as reflected in market share) in the long run, while R&D demonstrated a marginally significant effect. In addition, this study revealed that the interaction effect between marketing expenditure and R&D expenditure on brand competitiveness was marginally significant. These results demonstrate that companies need to invest in marketing activities to leverage the benefits of R&D in order to improve their brand competitiveness. As most companies often face budget constraints, maintaining investments in marketing and R&D is recommended to ensure sustained competitiveness in the long term.