Edit in JSFiddle

div {
    position: relative;
    width: 200px;
    height: 280px;
    border: 1px solid;
    overflow: hidden;
}

div:before {
    position: absolute;
    top: 0;
    left: 0;
    width: 75%;
    padding: 10px 0;
    text-align: center;
    color: #fff;
    background: #f00;
    content: 'nuevo';
    -webkit-transform: translate(-25%, 50%) rotate(-45deg);
    -moz-transform: translate(-25%, 50%) rotate(-45deg);
    transform: translate(-25%, 50%) rotate(-45deg);
}

<div></div>