The six largest political parties spent £36.7m during the regulated two-week period to 7 May last year, compared with £30.9m in the run-up to the 2010 election, the regulator's figures show.
The figure refers to spending by the Conservative, Labour, Liberal Democrat, UKIP, SNP and Green parties and does not include certain costs such as staffing or candidate spending.
The Conservatives spent £15.6m in the two weeks, while the Labour Party spent £12.1m and the Lib Dems £3.5m, UKIP £2.9m and the remaining parties both less than £1.5m.
A breakdown of the figures shows that spending on advertising by all of the largest political parties has fallen from a high of £16.1m in 2005, to £9.4m in 2010, and a little less than £7m in 2015.
Conversely, spending on mailshots and market research has risen during the same period, from £12.3m in 2005 to £22.1m in 2015.
Overall, the Electoral Commission said £39m was spent by a total of 57 parties and 23 non-party campaigners in the run-up to 2015, compared with £34m by 40 parties and 24 non-party campaigners in 2010.