A Thieving cape is the master cape of Thieving. The cape can be bought alongside the Thieving hood for 99,000 coins from the skill master, which in this case happens to be Martin Thwait at the Rogues' Den.
Like all skill capes, the Thieving cape gives +9 to all defensive stats, and also a +4 bonus to prayer if it is trimmed, which requires a separate skill at level 99. The cape is automatically trimmed if this condition is met.
When the thieving cape is equipped, players pickpocketing NPCs will have an additional 10% chance of being successful. This effect stacks with the bonuses from Ardougne Diary.
As of 1 September 2018, there are 28,940 current members that have achieved level 99 in Thieving.