I had no good guess what frog this might be, but Dick Bartlett helpfully pointed out that it looks like Pristimantis malkini, and indeed it does look just like photos of that species that I found subsequently.

Here is a complete list of the herps I saw in the wild on my 2014 MT Amazon Expeditions trip.

Pristimantis malkini has a particularly large head for a frog of this small size, which is evident here.

My Travelogues and Trip Lists page includes a complete list of the herps I saw in the wild on my 2016 MT Amazon Expeditions trip.

These frogs look quite different from the various more common Pristimantis in this area. So different that when I saw this one, it didn't even strike me as a Pristimantis at all; I thought it was probably a Leptodactylus species. Maybe this would be excusable if I hadn't seen this species on two previous trips.

Still, I feel the need to justify my poor frog-identification skills and/or memory. All of the other Pristimantis species I've seen here are far smaller, have proportionally far smaller heads, and are nearly always found up on plants (typically on leaves).