No, it's not an opportunity attack.

An opportunity attack is a specifically named game mechanic with specific rules set out for it (as found on page 195 of the PHB).

If the Sentinel feat was meant to be an opportunity attack it would say something like:

[...] you can make an opportunity attack against the attacking creature.

As it's written it's not counted as an opportunity attack because it doesn't say it is. What it does say is that:

you can use your reaction to make a melee weapon attack against the attacking creature.

No, it's not an opportunity attack.

An opportunity attack is a specifically named game mechanic with specific rules set out for it (as found on page 195 of the PHB).

If the third benefit of the Sentinel feat was meant to be an opportunity attack, it would say something like:

[...] you can make an opportunity attack against the attacking creature.

As it's written, it's not counted as an opportunity attack because it doesn't say it is. What the third benefit of the Sentinel feat (PHB, p. 169) does say is that:

[...] you can use your reaction to make a melee weapon attack against the attacking creature.