Working Families Tax Relief Act of 2015

Amends the Internal Revenue Code, as amended by the American Recovery and Reinvestment Act of 2009, to: (1) make permanent the reduction in the income threshold (from $10,000 to $3,000) for determining the refundable portion of the child tax credit, (2) eliminate the inflation adjustment to such amount, and (3) allow an annual inflation adjustment after 2014 to the $1,000 maximum credit amount and the adjusted gross income threshold amounts used to reduce the allowable amount of such credit.

Modifies the earned income tax credit to: (1) make permanent the increase in the rate of such credit for taxpayers with three or more children, (2) increase the credit for taxpayers with no qualifying children, (3) reduce from 25 to 21 the qualifying age for such credit, (4) revise eligibility rules relating to married individuals living apart and qualifying children claimed by another family member, and (5) repeal the denial of such credit for taxpayers with excess investment income.