This paper aims to examine the credibility value of sustainability assurance and the type of assurance provider on cost of capital. A large sample of international companies from the period 2007–2014 was used to develop our models of analysis. We find a greater decrease in cost of capital for companies that publish and assure their social and environmental reports. Thus, voluntary sustainability disclosures decrease the cost of capital. However, companies also have the opportunity to reinforce this decrease by providing an assurance statement, so increasing the credibility of corporate social responsibility information. In addition, the decrease in the cost of capital is significantly higher when such assurance is provided by a top‐tier accountancy firm instead of by engineering or consultancy firms; this result supports also the reputational capital of accountancy firms.