Block 21, Mixed-use entertainment venue in Downtown Austin, Texas.

Block 21 is a $300 million mixed-use development in Austin's Second Street District, comprising the W Austin Hotel and Residences, Austin City Limits Live at The Moody Theater, and 53,000 square feet (4,900 m2) of shops, restaurants and office space. The W Austin is the city's eighth-tallest building.

The project began in the early 2000s and transformed Austin's Second Street District from an underused area into a modern urban center. This development helped bring mixed-use activity and new investment to a part of downtown that needed revitalization.

Austin City Limits has been recorded at The Moody Theater since the venue opened in 2011; the show began in 1974 and is the longest-running music program on American television. The 3TEN Austin City Limits Live club, a smaller room in the same complex, hosts further concerts. The theater is operated by Ryman Hospitality Properties' Opry Entertainment Group, which bought Block 21 in 2022.

The complex has multiple parking options and sits on a central downtown location served by local transit systems. Walking is easy between the different sections, and multiple entrances provide access to restaurants, shops, the theater, and office areas throughout the space.

The complex integrates the W Austin Hotel and Residences directly within the same structure, blending overnight accommodation with entertainment and retail in one location. This combination is uncommon in Austin and creates a self-contained destination where visitors can stay, eat, shop, and enjoy live music without leaving the building.