There are three correct answers to this question, here presented in increasing relevance:

1. If you look at the US debt in dollar terms, the last president to reduce debt over one fiscal year was Harry S Truman, who did reduce debt in the fiscal year [1950-1951][1]. However, during his term, debt increased with 7.3 billion dollars, and your question of "Who was the last president to reduce debt" probably is intended as a question of what president reduced debt over his presidency as a whole.

2. So a more relevant answer is which was the last president to have lower debt when he exited office than when he entered it, and that was Warren G Harding, who reduced debt from [23,977 million dollars in 1921 to 22,349 million dollars in 1923][2].

 However, absolute numbers aren't that relevant in the first place, they aren't even adjusted for inflation. More relevant is how large the debt is as a percentage of the gross domestic product, as that gives you the most relevant information: How capable is the country of paying back that debt.

3. If you look at debt as a percentage of GDP, [the last president to reduce debt was Bill Clinton][3]. 

 Bill Clinton started with a total debt outstanding (which includes intra-governmental debt) of from [4.2 Trillion dollars][4] and a [6.5 Trillion dollar GDP][5] to 5.7 Trillion dollar debt with a 9.9 Trillion dollar GDP.

 So yes, the claim that Bill Clinton increased debt is correct, if you look at the number of dollars. But as a percentage of GDP, which is the important value, he reduced it.

There is a fourth possible view, by looking at who reduced debt in inflation-adjusted dollars. But that answer is less relevant than debt as a percentage of GDP and most sources claim this is Bill Clinton anyway, but I haven't bothered to find the raw data to verify it.

  [1]: http://www.treasurydirect.gov/govt/reports/pd/histdebt/histdebt_histo3.htm
  [2]: http://www.treasurydirect.gov/govt/reports/pd/histdebt/histdebt_histo3.htm
  [3]: http://en.wikipedia.org/wiki/History_of_the_United_States_public_debt
  [4]: http://www.treasurydirect.gov/NP/BPDLogin?application=np
  [5]: https://www.google.com/publicdata/explore?ds=d5bncppjof8f9_&met_y=ny_gdp_mktp_cd&idim=country:USA&dl=en&hl=en&q=us%20gdp