is Onsen the same as hottub in america?

or is there anything different about the custom of it or what it is?

thank you!

You must be kidding.

Here you go:
http://www.google.com/search?hl=en&q=onsen

Only similarity is that the water is hot.
Check out the information on this site or google onsen.

I have an outdoor hot tub w/ bamboo surrounding that we use every day in Marin county. We also have been to Machu Picchu's Aqua Caliente onsen where Shirley MacLaine supposedly had her out of body experience. It had luke warm cloudy water w/ sandy bottom. The Thermal Pool in Elsey Park in NT in Australia was similar with a lot of bat droppings & stunk. The one at Pamukkale in Turkey was about the same but a lot of slippery Roman columns that you can sit on..
But all are no comparison to the onsen in Japan.
If you have been to thermal holes in Carmel, Stinson Beach, Calistoga, Ojo Caliente in NM, etc., then you will understand what the basic onsen settings are like. Most are in nature settings(up in mountains, river/ocean sides), large hot water pool to move around (some are big enough to swim but exceptions) with rock surroundings, beautiful Japanese gardens, bamboo forest, etc. with sound of water running and views of the mountains, river,ocean, etc. from outdoor roten onsen. The water is from the deep earth. The real onsen is once through abundant & overflowing scalding hot water w/ cold water mixing & no recirculation or chlorine. The temp. is hot~105F(same as my hot tub) and very relaxing.
And civilized with ryokan/hotel rooms, great meals, etc. to relax and recharge.
Most are gender separated but there are many time reserved private. The more you pay, i.e. in-room onsen bath, the more like a hot tub size.

No, but ofuro is kinda ... like hottub with heating mechanism generating hot water into a wooden tub.

Onsen = natural hot springs from earth, some onsen waters are heated or cooled down but mostl is natural as it comes. I like all of the Onsen I've been to in Japan, people there revere hot springs in right way.
I don't like much of the hot springs in the US, they don't keep the water hot enough, exception is Grover Hot Springs in California side next to Nevada border, it's a pool but the spring water is satisfactory.
Big Island claims they have hot springs, what actually it is is that fresh hot volcanic rocks with active lava underneath heating up some of the ocean or natural water close to surface.

hottub = is hot water in a tub, like ofuro (Japanese bath tub) with hot water. You're probably refering to round wooden tub filled with hot water placed outdoors which became popular in the US back in the late 60's and into 70's, correct?

Amazinga, I like your knowledge, enjoyed reading your comments.
LOL

Hot tub is interchangeable with Jacuzzi/Spa.
Ours is a round plastic with/ redwood sidings & jets.
In Japan once we got a brand new surplus free from a local tub maker after much negotiation and my wife and I carried a cedar oval tub through the middle of Minami Rinkan main street, lol. We were young then. It was great that we had a regular American hot water heater to fill it and enjoyed everyday. But anymore they are all plastic in Japanese homes or hotels. Some high end onsen ryokans still have solid wood tubs or tubs like we used to have.
We will be soon heading to a natural rock onsen near Hoover Dam accessible by boat only. We need a ryokan with onsen just like in Japan in CA.

Hoover Dam! Do you ever run into snakes? And how is water there?

We like northern areas. Glenwood Springs comes to mind.