I recently was replacing a damaged outlet in a barn that is on it's own circuit from a breaker panel. I wired it correctly (black to brass, white to white, etc.) and plugged a polarity tester into the outlet to double check when I was finished. It showed reverse polarity; as I go around the barn plugging the tester into other outlets I find they are all reversed. I thought my tester might be incorrect, so I used my electric tester with the black and red wires that you use to test for circuit completion. I found that I only got current from neutral>hot and neutral>ground. I've been inside the junction box at the beginning of the circuit and everything is correct there as well. It should be noted that this barn also feeds a second barn (also showing reverse polarity). What could be the problem here? Could there be a grounding issue?
Here is a diagram of sorts:

[breaker box]
---------[junction box]
-------[receptacles (2) and a light]
------- [1 light]
------- [2nd barn]
----[junction box]
-----[3 receptacles and 3 lights]

You will get more answers and ideas tomorrow morning. Does the barn gets its feed from a main back at the house? In the meantime you may want to start back at the panel that is feeding this sub. What are the colors and polarity of the wires leaving the house, then the same as they enter the sub in the barn.

The feed begins at a utility pole with a breaker box at the main house about 50 feet away. It is a 30 amp circuit with 10/2 UF Romex going to the barn. Colors are black, white, and ground, it is all 120 volts. Once it enters the barn it enters a junction box (not a subpanel) and splits off to the other lights/receptacles and the second barn, all with 10/2 wire.

Probably night time now but what is the polarity to each of those colors on the 10/2 as they come into the that first junction box?

At the main breaker box, are the white and bare ground connected to the strap?

When the wires enter the first junction box are white and bare connected together?

It's been about a year since I have been in the first junction box, from what I remember all is correct. Black to black, white to white, ground to box and grounding rod.I've never opened the breaker box, mainly because there is a large wasp nest inside the box... I guess I can remedy that. Could an error in the breaker box causing the polarity to reverse? I've only changed a breaker once (and not in this box) so I have no idea.
I don't know if this could affect it but the wire from the breaker box to the first junction box has no ground wire (unless it was cut at the barn) and grounding doesn't begin until a grounding rod in the barn which is connected in the junction box.

If each connection at each box, beginning at the breaker panel through to the last outlet on the circuit, appears to be correct, and still shows a reverse polarity, there must be a mechanical problem, bad connection, broken wire, something is wrong.

Need to separate each connection, then start at the panel, measuring 120 volts beginning at the breaker to white neutral and between breaker and equipment ground, connect those wires and test that cable at the end of the circuit, and continue on connecting each segment of the circuit and testing the end each time, looking for 120 volts across black and white and black and ground. Should be showing 0 volts between white and ground at each junction.

If there is any problem it will show up using this troubleshooting method.

Will do. Thanks for all the answers..