The 36-year-old striker, who will wear shirt number 29, goes straight into the squad for Wednesday's South Yorkshire derby clash against Barnsley at Hillsborough on Saturday.
Howard began his career with Hartlepool in August 1995 and made 164 appearances for the Pools, scoring 32 goals, before joining Northampton Town in February 1999.
However, it was at Luton Town where Howard made his name, making over 200 appearances and scoring 96 goals in a five-year spell at Kenilworth Road.
Derby County was Howard's next club in July 2006 - the forward signing a three-year deal for a fee of £1m - before a £1.5m move to Leicester in January 2008.
Howard was named as Leicester's Player of the Year for the 2008-9 season before re-joining League One Hartlepool in the summer of 2012.