Stopping Tax Offenders and Prosecuting Identity Theft Act of 2015 or the STOP Identity Theft Act of 2015

Calls for the Attorney General to: (1) make use of all existing resources of the Department of Justice, including task forces, to bring more perpetrators of tax return identity theft to justice; and (2) take into account the need to concentrate efforts in areas of the country where the crime is most frequently reported, to coordinate with state and local authorities to prosecute and prevent such crime, and to protect vulnerable groups from becoming victims or otherwise being used in the offense.

Amends the federal criminal code to: (1) include organizations as victims for purposes of prohibitions against identity theft or aggravated identity theft, and (2) subject an identity theft offense committed during and in relation to tax fraud to a fine and/or up to 20 years' imprisonment.

Directs the Attorney General to report to the House and Senate Judiciary Committees on: (1) trends in the incidence of tax return identity theft, (2) recommendations on additional statutory tools that would aid in the effective prosecution of tax return identity theft, and (3) the status of implementing DOJ's March 2010 Audit Report 10-21 entitled "The Department of Justice's Efforts to Combat Identity Theft."