Do You Need to Have Advantage Against a Surprised Opponent to Use Sneak Attack?

Question

The definition out of rules of the sneak attack is

Beginning at 1st level, you know how to strike subtly and exploit a foe's distraction.

But following the rules, you can only use sneak attack if you have advantage on the attack or if another enemy of the target is within 5 feet and you don't have disadvantage on the attack.

Does this mean that if the Rogue catches an enemy surprised at the beginning of the first round, he cannot use the sneak attack unless he has advantage?

Since a Rogue with the Assassin Archetype at 3rd level will score all critical hits against a surprised target, and with the new rules the critical hits "If the attack involves other damage dice, such as from the rogue's Sneak Attack feature, you roll those dice twice as well."...

Answer 1

Normally, you would be correct; if a level 2 Rogue (or level 3 rogue of any other Archetype except Assassin) catches an enemy by surprise or takes their turn before the start of the targets initiative count, they would also need Advantage on the attack for it to apply Sneak Attack. A level 3 Rogue with the Assassin Archetype get's the Assassinate feature, however, which does the following.

From PHB pg 97

"...you have Advantage on all Attack Rolls against any creature that hasn't taken a turn in combat yet. In addition, any hit you score against a creature that is surprised is a critical hit..."

The first part is important because it already grants you Advantage, which means automatic Sneak Attack against any creature that hasn't taken it's 1st turn yet. This applies whether or not you have surprised your enemy, but if you happen to do so, your attack is also a critical hit. If you can surprise your enemy AND go before them in the initiative (because surprised enemies still roll Initiative and have a chance of going before you) then you A. Have advantage on the attack against them (Sneak Attack), and B. deal critical damage with all damage dice (Assassinate)).

Answer 2

To get a Sneak Attack, you require either advantage on the attack roll or an ally within 5 feet of the target.

An Assassin Rogue gets advantage when attacking a foe who has not yet had a turn in this combat (Assassinate feature) so you can choose to use Sneak Attack if you hit.

If an Assassin Rogue scores a hit on a surprised foe then the hit is automatically a critical hit (Assassinate feature). If the foe has not yet taken a turn then you can (and should!) choose to use Sneak Attack.

However, whether or not the foe is surprised has no effect on whether or not you get advantage.

Short answer: Assassin Rogues are deadly when they get the drop on their foes.

Answer 3

Sneak Attack (Usually) Requires Advantage

You can add sneak attack damage, "…if you have advantage on the attack roll…" or "…if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have disadvantage on the attack roll," (PH96).

Surprised Doesn't Grant Advantage

When you're surprised, "you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends," (PH189).

To Answer Your Question

Does this mean that if the rogue catches an enemy surprised at the beginning of the first round, he cannot use the sneak attack unless he has advantage?

Yes, that is exactly what it means, since nothing inherent in being surprised allows for sneak attack to be applied.