Villarcayo de Merindad de Castilla la Vieja (Spanish pronunciation: [biʎaɾˈkaʝo ðe meɾinˈdað ðe kasˈtiʎa la ˈβjexa]) is a municipality located in the province of Burgos, Castile and León, Spain. According to the 2004 census (INE), the municipality has a population of 4,031 inhabitants.