A dynamic DMA window mechanism can resize DMA windows dynamically by increasing one DMA window at the expense of reducing a neighboring DMA window. The dynamic DMA window mechanism can decide to dynamically resize DMA windows based on a request from a system administrator, based on a request by an operating system device driver for an I/O adapter, or based on a performance monitor determining such a resizing would benefit system performance. Once one DMA window has been increased by allocating a portion of a donor DMA window, device drivers for the I/O devices corresponding to the two windows are updated to reflect the new DMA window sizes.