The mixed 4 × 400 metres relay at the 2021 World Athletics Relays was held at the Silesian Stadium on 1 and 2 May.

[1] Prior to the competition, the records were as follows: Qualification: First 2 of each heat (Q) plus the 2 fastest times (q) advanced to the final.

[2] Italian national track relay team confirms the first place obtained in the heats summary.

[3]