Basics of the Reaper Rush
Reapers are the a very unique unit in Starcraft 2, serving little to no purpose in a larger army but highly specialized for base harassment. Requiring only a tech lab, Reapers can be very strong early game if you are not prepared, especially if the Terran player does something risky like build a proxy rax close to your base to get reapers out quicker. With proper micro, defeating the Reaper rush can be almost impossible, so good scouting and map awareness are essential to beating early reapers. Since both Protoss and Zerg require tier 1.5 units to reliably beat Reapers, it is important to stay on top of your macro as well if you find yourself needing to micro workers to keep them alive. The following sections should help you defeat Reapers as each of the three races.
How to Beat Early Reapers as Protoss
Protoss are not terribly weak to Reapers because of the way Probe shields work and the speed of Stalkers - which make short work of Reapers - but can still be very vulnerable to early Reapers if they choose to build a Zealot first or are confronted with a proxy rax strategy. Many Protoss players choose to skip their first Zealot and chronoboost a Stalker as soon as the Cybernetics Core is done. If you are unsure of your scouting ability or fear a rush, this is a safe strategy and a good opener against Terran anyway. If you find yourself confronted by Reapers before you have a Stalker, don’t panic - micro probes away as best you can while continuing to mine. If you have a Zealot, send it after the Reapers - there is no chance of it catching them, but it will keep your opponent busy until you have more defenses. If you can keep your first Stalker alive or get a Forge up for Cannons, you will be mostly safe from further harassment.
How to Beat Early Reapers as Zerg
Zerg have the hardest time of the three races defeating the Reaper rush because they have fewer options when dealing with them. Scout aggressively as Zerg - if your opponent goes fast Reapers or a proxy rax and you are unprepared, it may mean the end of the game. If you scout an early tech lab, get an Extractor early so that you can upgrade your Zerglings with movements speed - speedlings are fast enough to deal with Reapers, especially on creep. Until you have speed or enough Zerglings to fend off your attack, use your Queen to keep Reapers away from your vulnerable workers - be careful though, because a Queen can be killed by Reapers, and that is a huge blow. Consider building a second to transfuse the first, or begin placing spinecrawlers. If you can live long enough to build a Roach Warren, you will be in a good position to defeat further attacks.
How to Beat Early Reapers as Terran
Terrans have the easiest time defeating a Reaper rush because even though Marines lose to Reapers one on one, they are cheaper and faster to build than Reapers and can easily defend your vulnerable workers from within the mineral line. If you scout or suspect early Reapers, you can either attempt to mass Marines or quickly tech to Marauders - with Concussive Shells, they can stop a Reaper rush dead in its tracks. If you are under heavy pressure, a Bunker will ensure that your defenders stay alive during the Reapers’ raids - this is a good strategy against all cheese. From there it is easy to transition to standard Terran play and strike back.