Abstract: Experimental governance is increasingly being implemented in cities around the world through laboratories, testbeds, platforms, and innovation districts to address a wide range of complex sustainability challenges. Experiments often involve public-private partnerships and triple helix collaborations with the municipality as a key stakeholder. This stretches the responsibilities of local authorities beyond conventional practices of policymaking and regulation to engage in more applied, collaborative, and recursive forms of planning. In this article, we examine how local authorities are involved in experimental governance and how this is influencing their approach to urban development. We are specifically interested in the multiple strategic functions that municipalities play in experimental governance and the broader implications to existing urban planning practices and norms. We begin the article by developing an analytic framework of the most common strategic functions of municipalities in experimental governance and then apply this framework to Stockholm, a city that has embraced experimental governance as a means to realise its sustainability ambitions. Our findings reveal how the strategic functions of visioning, facilitating, supporting, amplifying, and guarding are producing new opportunities and challenges to urban planning practices in twenty-first century cities.