Designed by Sir Norman Foster, the central London monument includes a black stone tablet beside a blue pillar.
The names of 1,600 officers are inscribed in a book behind the glass. It includes members of the RUC officers killed on duty.
Although there are 28 memorials dedicated to individual officers in the UK, this is the first national monument.
The memorial was opened by the Queen, who laid a wreath before attending a reception with officers' relatives.
Film director Michael Winner began campaigning for a memorial after the death of Pc Yvonne Fletcher in 1984.
Tony Blair, Michael Howard and Charles Kennedy attended the opening. The Police Federation said the memorial was welcome but overdue.
Hundreds braved the rain to watch the ceremony.