Inclusion Criteria:

* Pathologically confirmed T or NK cell lymphoma at the enrolling institution. For CTCL, patients with stage IB disease or greater are eligible.
* Relapse or refractory disease after at least 1 systemic therapy except for T-PLL, LGL, or T-cell Lymphoproliferative diseases with JAK2 fusion.
* Untreated patients may be allowed after discussion with P.I.
* Age ≥ 18
* ECOG ≤ 2
* Measurable disease defined by:

* Lugano Classification for systemic lymphoma or
* Atypical and or malignant lymphocytes quantifiable by flow cytometry or morphology in blood or bone marrow or
* mSWAT \> 0 or Sezary count ≥ 1000 cells/μL for CTCL
* Previous systemic anti-cancer therapy for T-cell lymphoma must have been discontinued at least 2 weeks prior to treatment.

* Glucocorticoids aimed at controlling lymphoma-related symptoms are allowed as long as they are tapered down to 20mg or less by the time of ruxolitiib initiation
* Topical steroids for CTCL are permitted
* See section 6.2 Subject Exclusion Criteria for guideline regarding adjuvant and maintenance therapy for prior malignancy
* Patients must meet the following lab criteria:

* ANC ≥ 1.0/mm\^3 or ANC \>/= 0.5/mm\^3 (if patient has baseline neutropenia due to lymphoma), platelets ≥ 100 x 10\^9/L or ≥ 50 x 10\^9/L (if related to lymphoma), Hgb ≥ 8g/dL
* Patients with LGL or T-PLL are not required to meet a minimum ANC or hemoglobin value for eligibility
* Total bilirubin ≤ 1.5 x upper limit of normal (ULN) or; ≤ 3 x ULN if documented hepatic involvement with lymphoma, or ≤ 5 x ULN if history of Gilbert's ; AST and ALT ≤ 3 x ULN; ≤ 5 x ULN if due to lymphoma involvement
* Creatinine clearance ≥ 30 mL/min; creatinine clearance of 15-29 mL/min will be allowed as long as baseline platelets are ≥ 150 x 10\^9/L
* For patients with positive hepatitis B core antibody or surface antigen, hepatitis B PCR must be negative and prophylaxis with entecavir or equivalent is required.
* Patients with HIV are allowed provided that they are on anti-retroviral treatment with no active infections.

Exclusion Criteria:

* Any serious medical condition, laboratory abnormality, or psychiatric illness that would prevent the subject from signing the informed consent form.
* Uncontrolled concurrent illness including, but not limited to, ongoing or active infection, uncontrolled diabetes, clinically significant pneumonitis, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.
* ECOG performance status \>2
* Prior therapy with ruxolitinib
* Receiving systemic therapy for another primary malignancy (other than T-cell lymphoma)

* Patients with more than one type of lymphoma may be enrolled after discussion with the MSK Principal Investigator
* Adjuvant or maintenance therapy to reduce the risk of recurrence of other malignancy (other than T-cell lymphoma) is permissible after discussion with the MSK principal Investigator
* Women of reproductive potential† must have a negative Serum ß human chorionic gonadotropin (ß-HCG) pregnancy test.

* A female of reproductive potential is a sexually mature female who: has not undergone a hysterectomy or bilateral oophorectomy; or has not been naturally postmenopausal for at least 24 consecutive months (i.e. has had menses at any time in the preceding 24 consecutive months).