No, they aren’t. And the below video is the best way to explain what the difference is exactly.

Video credits to Alan Thrall. It talks about the difference between the RDL and Stiff-legged deadlift.

More in depth answer:

Many say that both of these deadlift variations are just conventional deadlifts that incorporate less knee flexion to place more stress on the hamstrings, this is, in a way, true - but it isn't the whole truth.

Differences between the Stiff legged and Romanian deadlift:

Stiff-Leg deadlift

• Help strengthen the lockout of a conventional deadlift
• Keep your knees tracking out and pull through the stretch
• When doing the stiff legged deadlift, get rid of all leg drive and focus on pulling with the hamstrings, lower back, and glutes only (with the back neutral and chest up)
• Don't bounce off the floor. Start every rep from a dead stop on the floor and make sure to always lock out at the top
• Resembles the conventional deadlift more and every rep starts from the floor
• Uses very slight knee flex
• Usually done for strength

Romanian deadlift

• When doing the RDL focus on getting a stretch in the hamstrings for every single rep to maximize muscle tearing (the good muscle tearing - micro tears).
• Try not to let the barbell touch the floor at the bottom of every single rep. Also, try not to full extend to the top of the movement for every rep either - maintain hamstring tension for every rep.
• RDLs can be done from a deficit to increase ROM (placing more stress on the hamstrings
• RDLs can be started from the rack
• Reps typically start from the hip (while holding the bar above the knee)
• Usually done for hypertrophy (muscle growth)
• Uses a small knee flex