B-737 First Officer reported severe turbulence during cruise resulting in an altitude excursion and near over speed. A Flight Attendant in the cabin was injured during the turbulence and the flight continued to destination.

Narrative

Encountered unforcasted severe turbulence at FL 250.On the flight from ZZZ1 to ZZZ2; there were multiple reports of turbulence along our route of flight at various altitudes. No convective activity was present; and it was a relatively clear evening. I was the Pilot Flying. Our flight plan called for an initial cruise at FL310 until ZZZ VOR and then a climb to FL370. The first third of the flight was relatively uneventful with the ride fluctuating from smooth to light chop. During this time our Flight Attendants were able to perform their service; while the seat belt sign remained on to keep the isle clear. As we approached ZZZ; ZZZ3 was indicating turbulence (light to moderate chop) from FL250 up to and including FL370. Due to the aircraft weight and other pilot reports; the Captain and I elected to descend to FL250. After leveling; the ride still consisted of light chop; and we advised the Passengers and Flight Attendants to remain seated. Nine minutes after level off; the aircraft abruptly experienced large pitch and attitude changes resulting in the immediate disengagement of the autopilot. The ride was so rough that I found it difficult to focus on the instruments and it was a continuous battle to keep the wings level and keep the aircraft from over speeding. The Captain requested lower and the turbulence did not let up until passing FL230. We leveled at FL190 and began to assess the situation. We were informed by our Cabin Crew that our B Flight Attendant was injured; as she was standing at the time of the incident but was stable. The D Flight Attendant stated that a Paramedic; and possibly a Nurse; was providing care but did not feel her condition warranted a diversion. We contacted medical services through the Company; and they agreed to have Paramedics meet her at the gate in ZZZ2. I want to applaud the Cabin Crew as they performed wonderfully following the event. They provided the Flight Crew with good information and remained calm and collected. The rest of the flight was to ZZZ2 was uneventful. Just a reminder to the flight attendants that if they are briefed to remain seated to continue to do so until after they have been given the all clear.